Make more filters and restrict no email accounts.
Also, like other websites restrict the use to use an email in 2 or more accounts. (It doesnt hurt to make another email for a alt?) This will also prevent many useless alts.
Another idea, if a bot is detected auto-IP ban this account. Robux scammers dont deserve other chances. This is easy because there could be THOUSANDS of bots and they will all be removed very easily.