Please disable the following two options:
- Dashboard > Settings > General: Anyone can register
- Dashboard > Forums > Settings > Features: Enable User Registration
If you don't need registrations, definitely turn both off.
But if you need registrations, there are a lot of ways to limit spam and fake accounts.
What kind of protection have you tried (and failed) and what kind of abuse do you have? Fake acccount with profile spam? Spam posts?