"it keeps coming up because you haven't paid me, yet." is the correct response. You can't let people jerk you around as a freelancer. You have to eventually cut off anymore work until you are paid.

You shouldn't release the source until you're paid in full. Also, google "fuck you, pay me" for a humorous but insightful talk about contracts. I think the person giving the talk is named Mike Monterro, but I could be wrong.