Who cares if they are sick of you or not?!! Are you trying to maintain a relationship with these people or is this a business transaction? Its business and they need to rectify the situation in a respectable manner whether you are being a nice guy or an arsehole.
You need to put your foot down and be firm. Follow the suggestions outlined in the second post of this thread as it sounds like a very good tactic and will generate a solid platform for future action.
You also need to PM me with the name of this company - just for my own reference. I understand you not wanting to post it for fear of libel. Good call.
Scott
Consistency is a good thing....unless you're consistently an idiot.