This was meant to be included with the above post of mine...
I would have outlined that in the proposal, in a scope of project section, and made it clear that they understand what they are agreeing to.
But if this is still going on, call a meeting, or if you are uncomfortable with that, call them over the phone. In either instance, you want to tell them that though they may feel or have interpreted that the original agreement was to have this specific task completed every time without additional charges, but the proposal outlines what the actual responsibilities were and this is not the case. You can say something like, "I really enjoyed working with you on this, and I did resolve this issue a few times already, but it's not feasible to do time-consuming work without being paid."
I liked your second post, it was well worded. That sounds like a great way to deal with it.