"Look, I hate to say this, but I'm running a business, and I can't afford to spend an unlimited number of hours doing free customer support. A certain amount of that was budgeted into the contract, but we've already gone past that. I'd be glad to continue answering these small questions for you, but it will have to be at my usual hourly rate, or we'll need to work out some kind of contract for that service. Of course if you find a genuine bug covered by warranty, that's a different matter... but be prepared to pay if it turns out to be user error/education/configuration work. I like you guys, but I need to work on my other customers' projects, and I need to eat."
Heck, the Free Software Manifesto said quite clearly that even when software is free, support should be paid for. You need to pay the piper on one everend or the other, or learn to play for yourself.