Unpopular #Mastodon admin opinion, but I really think that, unless you are a small instance that’s just starting out, you shouldn’t enable open registrations all the time on your server. Either switch to an approval-based registration process or to an invite-based one. Or if you want to have open registrations, only do so during a specific time frame each day, like witches.town did.
Everything else is just an invitation to transform your instance into a spamming machine.