Keep spam under control.
Spam is a very common problem that admins need to deal with and which we aim to improve in core, but there are some actions site admins can take to mitigate the problem.
Install an anti-spam plugin
There are several available on the community
Change the registration url
Some have said they have good success changing the registration URL, since the spammers are naive bots and can no longer find where to create the fake accounts.
Disable open registration
If the flow of registrations is low enough, you can vet all users that come in to keep the quality of interaction high.