further to Deadly's advice to block the IP addresses of certain hosts. A bit of careful filtering and experimentation can refine this method somewhat. A slightly more specific list of blocked hosts that get rid of most 'bad' T3 or higher sharers is:
38.118.*.*
38.117.*.*
66.54.74.*
66.54.73.*
66.54.72.*
66.54.76.*

Note: not all hosts with 66.*.*.* IP addresses are T3 or above.

I didn't find I needed to block the 68.*.*.* hosts, but since all this must depend on what you search for perhaps it is needed.

Someone with more time on their hands could compile a better list.

If you get the specific IP address of a bad host you can search
http://ws.arin.net/cgi-bin/whois.pl
to find out some more information about it. Usually no more than which internet service provider they use. FYI the 66.54.7* addresses all go to this ISP:

OrgName: NHI Networks
OrgID: NHINE
Address: 530 W 6th St.
Address: Suite 300
City: Los Angeles
StateProv: CA
PostalCode: 90014
Country: US

Perhaps they know what's going on?