You would need to ban their IP address, which can be obtained on their profile. Since you've deleted them, you won't be able to get that IP address anymore, until they have come back. There is the possibility of you having an old "registrations.csv" on your computer somewhere. Possibly in your downloads folder on your pc. In that case, Remote IP's are listed and you could get it that way.
If you do get the chance, you click on manage site, settings, manage settings, login and registration, and towards the bottom, you will see a place to block an IP address, and you just put it there, and click save.
