@Alison27
Unfortunately this is something that guests will do and there's nothing you can do to circumvent it.
Normally either guest or host will send a booking change that the other party will have to confirm.
If the days they want to stay are less than your minimum booking days, you could for ex. quickly change your minimum to 2, send them the change but with the price of your real minimum days, accompany it all with a message explaining this, then when sent, don't forget to re-set your minimum day settings.
And you never give money back directly ever, it's normally all done through the system as Airbnb handles the money. This is all explained on the website. Uninformed hosts have given guests money back in cash and then the guest went and additionally asked for money back through Airbnb. And since transactions outside Airbnb aren't allowed, the guest ended up getting double the amount.