At Admin Panel -> Members -> Member Settings, you have a variety of banning options. You can ban proxies, ban malicious IPs identified by stopforumspam.com, block all unresolvable host names (dangerous), block email domains, block host names, block city names, or simply block IP addresses either from the whole site or just from registering.

You can find IPs by editing the profile of a problem user to find their recorded IP, or by editing a link to see the submitter IP. You can also find IPs of people rating listings, by clicking the 'show individual votes' link on the listing details page.

Note that you can ban wide IP ranges. Banning the IP 12.25.36 will also block 12.25.36.43.7, etc... if what you type in the IP ban box matches the begining of the IP, the IP will be banned.

You can also select if you wish to ban these IPs from the whole site or simply stop them from submitting listings/comments and registering. Since in banning an IP range you may often block a few innocents, it may be safer to let them continue viewing without being able to submit.