Nothing on a rental bike should be removable or adjustable by the renter. Everything including wheels and seat posts should at the very least need a wrench to remove/adjust, and ideally should need a special tool with a security fastener.
Yes, that means you'll need to adjust the seat post height when you send the bike out. That shouldn't take more than a couple of minutes per bike, and if you can maintain a bike then you can surely manage that. And if you don't have a bike that fits them, you don't (can't) rent them a bike.
This also protects you against theft of parts. With quick releases on wheels and seat posts, thefts of bike parts is common almost everywhere, and if you've not yet lost anything then you've been extremely lucky.
Of course preventing renters from fixing a puncture might be inconvenient for them. But you can't trust your renters to know how to fix a puncture without causing damage; and for cycling around town, having to take the bike back to be fixed is only a minor inconvenience at most. This would be different if you're renting mountain bikes for long treks, of course, but then you're talking a very different target market, and usually a trek like that has a guide/leader who you work with anyway.