Within this question there are actually two inquiries. One concerns the theoretical pinning of the Biblical prohibition of revenge (see Vayikra 19:18); the other concerns the practical application of this concept in this case. Briefly, I will attempt to deal with both.
Two different yet clearly connected prohibitions are actually derived from this verse. One -- recorded in Rambam (Maimonides), Sefer HaMitzvot, Negative Commandment 304 and Sefer HaChinuch, mitzvah 241 -- is the more direct one relating to revenge, namely the prohibition of hurting someone simply because he/she hurt you. T.B. Yoma 23a presents, as an example, Person A refusing to lend something to Person B because the latter previously refused to lend something to the former.
The second commandment included in this verse – recorded in Rambam (Maimonides), Sefer HaMitzvot, Negative Commandment 305 and Sefer HaChinuch, mitzvah 242 – is of a more indirect nature and is usually referred to in English translation as bearing a grudge. The gemara gives as an example the case when Person A does still lend something to Person B but makes a point of mentioning that this is unlike Person B who did not lend Person A an item when asked. Sefer HaChinuch points out that the connection of both these prohibited behaviours is the desire to hurt the other in the same manner that this person was hurt. Person A was hurt by Person B because the latter did not lend the former an item. Person A’s motivation for either now not lending Person B an item or lending the item but only after making a point of his/her better behaviour is simply to treat the other badly and/or hurt the other out of grievance. This is what is the essential wrong in both these cases and is at the root of both prohibitions..
In the case of these mitzvot, the nature of this motivation is, thus, of great importance. The previous verse, Vayikra 19:17, informs us that we have a responsibility for our fellow Jew and, as such, we have an obligation to rebuke them, to foster better behaviour in our fellow Jews. If there is a positive purpose in undertaking an action against another, the action does not fall under these prohibitions. So if you say something to another when lending this person an object after he/she did not previously lend you an object, but your objective was not to cause pain but to instruct the other in correct behaviour, with a goal that this instruction will be adopted, you have not violated these commands. It’s the same as hitting another. If you do so because he/she hit you, that’s revenge. If you do so to protect yourself and to stop the other from hitting you, it is not.
As such, it is specifically when an action of a revengeful nature is undertaken to solely cause pain to the other that we encounter a violation of these commandments. An action of this nature undertaken for a positive purpose – whereby the motivation is this positive purpose – does not fall into these categories of sinful revenge. That is why this very same gemara in Yoma states that a Torah scholar who is not vengeful is not a true Torah scholar. Rambam, Mishneh Torah, Hilchot De’ot 7:13 explains. When a Torah scholar is belittled, especially in public, it is not his/her personal status that is diminished and/or attacked; it is the very honour of Torah itself that is challenged. In this regard, the Torah scholar must take action for the honour of Torah. The motivation is not personal hatred and/or desire to hurt. The motivation is furtherance of Torah. When action that seems to be vengeful is undertaken for a positive purpose, it is not included in these prohibitions. One must, of course, ensure, though, that the true motivation is this positive purpose and that one is not just finding a rationalization for spiteful revenge.
.So, on the surface, it would seem that this boycott of ones boycotting Israel would be acceptable as the purpose is not just to cause pain to the ones who instituted the original boycott but also to try and effect a positive change in, at least, attempting to thwart the original boycott.
There is, though, a further problem. Is this response truly in keeping with principles of justice and the parameters of how we are to act in response to an act of aggression (which this boycott is)? The answer in this case may clearly be no. The ones who instituted the original boycott against Israel are not the ones affected by this boycott for Israel. There is even a question whether the ones affected by this boycott on scotch whiskey can even influence the ones who started the whole matter by boycotting the purchase of books published in Israel. It seems that the real motivation may be to hurt someone because someone hurt Israel. This would even be a case worse than the simple parameters of revenge. In those cases, we critique a motivation to hurt another because he/she hurt you. What would clearly be even worse would be a desire to hurt someone, almost anyone, because you were hurt. That may be the case here. Is this a case where we are so upset with boycotts of Israel that we just want to do something to express our anger and hatred of these boycotts? If so, it would be clearly wrong and any arguments for such an action would have to be seen as only invented to find some means of justifying the action. This is the core of the issue. Is there a real, positive purpose to this action or is it simply a way of expressing anger and hatred? If the former, it could be justified. If the latter, it would not be.
(It should perhaps also be noted that a further argument could also be made to justify this reactionary boycott in that it would seem that the original Biblical prohibition is solely on taking revenge on fellow Jews. I do not wish, though, to pursue this argument as it, in turn, raises the whole issue of why there should even be a distinction between Jews and non-Jews in this regard or, in fact, in regard to any ethical standard. This further issue, though, is a most complicated one – generally often misunderstood – and beyond the parameters of this question. It should be stated, though, that while this distinction does exist, an argument for vengeance based on such an argument, would still be seen as fundamentally weak for, while technically a distinction between Jew and non-Jew has some basis, the character trait of being a vengeful person is still clearly perceived to be a most negative one. As such, the literature clearly maintains that it should not be part of our base personality in dealing with anyone. The complex question that then arises, as such, is: why would the Torah then clearly demarcate such a law, in any way, as only applying to fellow Jews when, in fact, such behaviour is expected to be rejected universally? This is the question that unfortunately we are not able to pursue at this time.)
