No, altering a reservation is not a loop hole for the guest to get out of paying the remainder of the trip.
If the guest proposes an alteration, you must accept the alteration before it goes into effect. If you decline their request for the alteration, they will have to cancel the rest of the reservation themselves and get what refund they entitled to under your posted cancellation policy. There is still a potential problem. Here is an example:
Guest A is three days into their 10 day reservation. They tell you on day three that they "want to cut the stay short". They want a refund of all the nights they won't use. You reply "This is peak season, I will refund you any days I am able to re-rent the place, is that OK?". Guest A says no, they want a full refund of their unused days. You are now on the horns of a dilemma. Since guest A has already stayed a few days they will be able to write a review. If you make them unhappy at this point, just by following your posted cancellation rules, they might just leave you a bad review as revenge.
Airbnb may under very limited circumstances, if the "revenge" review is truly outrageous, remove it. But if the review is just kinda bad, they will very likely leave it be. If you are a long time host with a bunch of future reservations coming up, this one bad review will be weighed against all your prior good reviews. Then it will quickly get buried under subsequent guest reviews. BUT, if you are new, this one bad review can be a poison pill and potentially ruin your new business.