Your main point of leverage is always going to be the willingness to drop them. In your case this doesn't appear to be possible, so you have no control. Us non-nasty, reasonable people can't expect to win against people who've been practising this their whole lives.

Suck it up now, but get an iron-clad agreement from your management to never again court or sign up a client unless they're "nice". And bake it into your company culture. Yes this will limit you to only a section of the market, but unless you're realistically looking at "world domination" you can afford to be a bit picky.