How to stop bike renters overextending seatposts?
I assume they are extending the seat post that far because they need it for an appropriate saddle height.
So the solution is simple: Offer bikes with larger frame sizes or at the very least equip the bikes with long seatposts which can be extended far enough.
Any other solution would force your customers to ride with a too low saddle.
If you still want to make sure that they can’t be extended too far, just in case a giant with long legs comes along, there is a solution I can think of: If you have seatposts where you can see through the tube (common on cheap steel seatposts) you could try to attach a string (or steel cable) somewhere at the bottom bracket, thread it through the seat tube and seatpost and tie it to the saddle rails. Could lead to some noise though if the cable rattles inside the seat tube.