OK, I'll bite

What did you do to upset them? I've used more than a dozen agencies.
One of them "tried it on" with iffy contracts, which I refused to sign
and they modified them EXACTLY as per my request; One of them was a
really lousey payer (some payments being 6+ months late) due to
clerical errors. I would have made a big deal of it, but, due to more
clerical errors their end, they accidentally over paid me about £6K
By and large, I'm in favour of agencies. I make more money for less
effort with them than without them. Show them the respect that you'd
hope they should show you (and, mostly, do) and you are unlikely to
have any problems.
So, tell us, why have the agencies blacklisted you? What did you do to
upset them?