How can I limit users to one account each? (Or, how can I detect users who have multiple accounts?)
You can not. Only limitation is Useraccount with one Emailadress. But if the same User Logs in with a different name and a disposable email adress, you can not verify if it is the same User. Something like undisposable will help. But it's not the solution.

Also if 200 Users use the same Proxy/same IP, you will limit them.