Do it objectively. Find and focus on undeniable reasons, not your gut feeling. "I don't like your ethics" is subjective and is asking for confrontation. "The

Do it objectively. Find and focus on undeniable reasons, not your gut feeling. "I don't like your ethics" is subjective and is asking for confrontation. "The number of complaints registered against you is x and it harms my company to be associated with that. So it's a shame, but we're not going to be able to sign contracts for further work. Don't worry, we'll finish what we've already agreed to do so far but after that we're going to have to call it quits" is objective. Note that you're not judging them, nor are you even implying that the complaints to be valid. It's just a business decision based on the number of them, and who can control that?