Let them go to voat, it was specifically founded for the alt-right, same as gab.ai as a Nazi twitter clone. They're all alone out there and don't have any influence anymore... the problem with T_D specifically is that their users lesk out to other, normal subreddit and spew their stuff there. Ban the subreddit, ban all the top commenters/submissions, their admins and forward the dirty stuff to law enforcement.
Except it wasn't. It simply experienced more and more waves of users from banned subreddits (which were mainly alt-right related ones) coming in and eventually making an hard right turn on the main website culture, driving out most existing center/left users. It did serve me as a lesson on what external influences can do to existing communities tho.
Contrast that with something like https://voat.co/ for instance. At the moment I can see only one (1) post in the entire front page that's not alt-right politics. Special mention to the highly upvoted "If women have the right to terminate unwanted life because it's in their body, do we have the right to terminate unwanted illegals because they're inside our country?". I don't expect most advertisers will want to touch that with a one parsec pole.