Need Rules to Block Host

Hello,

I have been receiving referrals traffic from 3 websites and that flood apache with errors and cause server to slow down. I want a rules where i can add host to block request from them. I searched online many people are facing problem with ib.adnxs.com and the apache gets flooded.

pm.5188bh.com
exchange.admailtiser.com
ib.adnxs.com

More information here:

http://serverdown.ttwait.com/que/605343