If one recruiting firm won't represent you as a candidate, it's no loss, as there are thousands more that will - and recruiting firms aren't comparing notes with other firms on who took a counteroffer.

It's a threat that is totally unrealistic.

As a recruiter, I'd actually say that technologists are more likely to be able to damage a recruiter's career than the other way around. If word gets out about a recruiter not behaving properly, that word can travel much faster in the worker space than word travels in the recruiter space.