Domain block lists (aka burner, anonymous, disposable email domains), some IP blocks, a lot manual blocking. Creating a new email address is easy so unless you

Domain block lists (aka burner, anonymous, disposable email domains), some IP blocks, a lot manual blocking. Creating a new email address is easy so unless you require a phone number or credit card people will find a way around. Just today one person registered 16 times in an hour. We've also seen classrooms signup almost in parallel (within 5 minutes) or people asking their (alumni) friends from all over the world. It's part risk of offering a free trial I guess.