Doing the silly unvoting is clearly not even necessary to maintain proper diversity, and some users picked up the trend without really thinking about the harm it could cause to the platform. As is the case with life in general, going nuclear against someone who makes a mistake is probably not the best solution. Fair warning with fair and temporary punishment is how people learn, by suffering the consequences from their mistakes, but while also having an opportunity to learn from it, mature, and leave the consequences behind after their time is due.
@zombee, while is it true users (me included) should have inform you about the loophole, instead of adopting the trend without thinking about it, it is also true that forbidding the use of the platform for life might not be the best measure if the goal is having an active and growing community. I understand the fact that the silly unvoting trend should be punished, but not for life. One silly mistake is not deserving of that permanent punishment.
I believe the 2 weeks ban you are proposing now with vote diversity checking is fair and will clearly serve as an example to others.