It was a jailable offence on PC Classic (I'm aware that Classic is not Zone).
The trouble I see with making it illegal on Zone now would be the massive player count. Enforcing such a rule would be difficult, and an automated system doesn't account for ever possible scenario where someone could be innocent or guilty.
You could, however, decrease the amount of points toward a PK score you get when killing the same player too often. I don't know how PKing effects points or score, or it there is even a damaging issue at all, because I don't play Zone; however, if there is a point system and boosting is an issue it could alleviate part of the problem by making the effects of boosting less damaging. Or, all of this already exists and there may be nothing that can be done further without ruining game playability.
When I played Zone on PC, it didn't matter if someone had a high kill streak because there was a bounty system. The more people they kill, the higher their bounty and the more money you would gain from killing them. I can see getting a high bounty and purposefully dying to a friend to give them money a small issue. That was rare on PC because it took too much time to get the bounty and the bounty was easy to lose to someone other than the person you intended to give it to. AKA it was a waste of time.