I'm not sure how I feel about this. It will almost certainly not free any of those in russia that are currently mentally enslaved to this cult (indeed it will probably cause them to double down in their commitment), but it may well help prevent future victimization from taking place. This also has me wondering how truthful the cult's frequent line that when under ban they often have more growth than in countries where they're free to practice. That seems feasible because it would require even more deceptive recruitment tactics, as opposed to the slightly more up-front (though still far from honest) recruiting in other countries. But it seems most things that the cult says is either a lie or wildly deceptive, I'm not so sure.
I wonder if something else might be more effective - instead of banning "extremist" groups (which is something that can be somewhat subjective to define, and will always be debated by such groups) they should ban any groups that push for deceptive recruiting tactics and do not provide full disclosure to prospective members up-front. This would be reasonably easy to demonstrate to be the case with JWs and a statement saying that you're banning a group because they use deceptive recruiting tactics and have refused to be up-front in recruiting after being given a warning sends a very different message. They can no longer claim persecution as effectively (especially if documented evidence of said deceptive recruiting is provided to the public) because they're being punished for what is essentially fraud. It would also do a lot to warn the public away from them - being banned because you're extremist can be read as being banned because you really believe in your cause which some see as a virtue. Being banned because you deceive people into joining your group has no up side to it.
It might be harder to implement something like that, though, in a legal setting. But it would probably still cover everyone that they want to target - extremist groups and cults all use deceptive recruiting.
