Some folks write notes out of genuine courtesy and some as a set up for anything goes when the party gets too raucus or goes way too long. Fortunately, it seems young folk these days like to drink at home then go out which is better than the reverse.
If your bylaws do have a cut off time you might mention that in a courtesy note back to them. If they are nice people you can roll with it more easily than if they are rude. If this one goes ok and doesn’t become a too regular occurrence you will probably be happy but if not you have your response as part of your cannon fodder.
On the night I appreciate some folks annoyed by another’s party may be a little reticent to contact them if they think they might not respond the way you want, so do feel free to call the cops who are generally happy to attend if they aren’t busy elsewhere and do so without naming you as the caller.
If it does go pear-shaped, keep in mind what a bouncer told me he does which is to use his ATM strategy: Ask them, tell them, make them to have it addressed and not repeated.