Of course there's no way an agency that gives you a bait&switch can force you to take the girl, if you don't let her in to your apartment/hotel room there's not much they can do apart from refusing to send you another girl

The problem is that it usually takes at least an hour from when you call the agency until the girl arrives so you could easily end up wasting hours waiting for escorts that aren't the ones you booked if you're unlucky.

Prague is cheap, but to be honest I can't think of any agency with a good reputation that can deliver an escort who'll tick all your boxes for 100 Euro. I think if you go down the escort route you'll either have to take your chances with the questionable agencies and hope for the best or increase your budget.