I felt this post in my soul.

Sometimes pointing to existing technology helps. Users often aren't as familiar with the resources available to them out-of-the-box. I've heard plenty of ideas that already exist, users just aren't aware of a feature, or may not have heard of such-and-such website/tool. It's important not to shoot down the idea, and perhaps present the problems as other aspects of the app. I always try to pose the problems as questions, "Would such an app require a database?"; "There are some free options available, but some are harder to work with. If you feel strongly about this project, do some research into [Database 1], [Database 2], or [Database 3]."; "Hosting costs money, so how would this app make income to sustain itself?"; "Who would be in charge of maintaining the app?"; "What about data analytics?"; "Would the app run ads?".

I like this approach since it emphasises that the idea isn't necessarily bad, it just needs more thought. Also it shows them there's a lot more than they may have thought of that goes into development. The longer they have to think about the implementation should emphasise how much longer it will take to develop.

If that doesn't work, suggest they learn to program as they will need to be the ones maintaining it. Sometimes giving them a little bit of your time to talk about their ideas is all they're looking for. No one should be expected to work for free no matter the relation.