I thought there was, but I can't find it now either. Looking at a 3.2 install, it's a tab called "IP Banning" under Settings but it's not there in 3.3 (I never used it, so I didn't notice that until now).
You might consider my plugin,
MTAutoBan, if you have an Apache based webserver. It doesn't support manual banning, however, but it might suit your needs.