How about answering "Sorry, I don't have time to fix this over the phone, but how about you invite us to dinner and I'll look at your computer then?"

I have a super-helpful computer-techy dh too... But he's getting better at asking for help himself for other things, and making a point about how it's great to "trade knowledge". Or asking if they've already called support (they never have, but at least it plants the idea that it's possible).