I still remember @theymos post about his detective work to compile the whole alts by IP address, but I'm unable to find the post. This mean it's possible for @theymos ban the account by IP address matched with the ban evaders.
Theymos can, but won't use that approach, it can cause many mistakes and members that aren't the ban evaders themselves may get banned, that is why the forum uses the proxyban method to ban members who share evil or banned ip's, but this takes place when they register, and getting whitelisted or paying a small fee solves the problem.
Are you sure Theymos ever said he will compile alts by IP addresses? Theymos answers just once out of a thousand times his attention is needed, surely he would not involve himself in any detective work that will be more time consuming.