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.

I had the fake users issue with a SaaS I built. They abused the free plan offering by creating multiple accounts in parallel. I started detecting temporary emails and blocking them from signup - this solved the problem 90% for me. People can still create accounts with new gmail addresses but its way more effort than temp mail services, so many won't. So I def recommend blocking disposable email. For starters you could use publicly available block lists (search github) but those are not as comprehensive/up to date. If you want to look at my solution, it has become a service of its own, check out https://istempmail.com

And note: no matter what measures you put in place, there will always be some people determined enough to find a way around them. So use simple solutions and accept a few grifters on board.