The only thing I can think of is if you happen to be online while that specific guest is on.. Go on your Forum > Click Administration Panel > Under "Who Is Online" locate the guest > Copy their IP Address Then just block their IP by going to Users & Groups > Ban Control > Ban an IP Address
As Valoish said, the only thing I think you can do is wait until they are online and ban their ip address. Bare in mind, however, that ip addresses change from person to person every now and then, so the same person may not always have the same ip address and thus someone else who gets that ip address at some point that may want to visit your forum and will be banned also. This doesnt occur often I believe, but it can happen. Therefore it is advised to use this measure wisely and carefully and as a last resort only if you have to.
Banning the Ip address will not make any difference at all as the Ip address is only banned when the member logs in the forum that is linked to an account.
so The ip address is not banned until the account is made
lal I get it. Honestly, I was just looking into some auto-banning code. Like if someone clicks "I do not agree to the terms of service" per se, they would get banned lmfao