This feature already exists.
The only thing that would prevent them from being BLOCKED FROM SIGNING UP as new accounts from the same IP is if:
-
the IP belongs to a user at trust level 2, if an IP belongs to a user that is already at TL2 we assume that IP can be trusted
-
the IP belongs to a staff member, if an IP belongs to a staff member we assume that IP can be trusted
basically, all the IP use must be from new users, for it to work. (And as @cpradio pointed out, the threshold needs to be at a reasonable value)