One guy called me a while back (in my role as an employer) who, when I politely said that we weren't interested because we always recruit directly, told me that he had been responsible for someone leaving our company a few months ago and he would now actively try to "poach" more people away until we agreed to use him.
Needless to say, we won't be doing any business with him or his company even if we decide to use agencies in the future. If it weren't for UK libel laws I'd mention his name as a warning to others.