I had a couple of ideas that made me chuckle but first lets say, just throw their stuff on the ground, or heave that expensive towel that has to be returned over board. lol sorry that was bad.
1. sit in the chair, when they walk up to you and say " that is my chair." look around at the chair, ask their name and say nope don't see it on here.
2. sit in the chair when they come up and say "that was my seat" kindly reply yes your right it WAS your seat. muwahhhh
3. sit in the chair when they walk up and say whatever they decide to say. Politely lift your sunglasses up look at them burp, slur your words and say something about them moving because they are blocking your sun, and ruining everyones day.
4. sit in the chair when they walk up to you and say something reach over grab a post-it note write on it " To bad, finders keepers looser weapers" and hand it to them.
Note if any of these things cause an altercation of any sort throw the person over board and or make sure that you say in a tone so everyone hears but is polite " I am sorry I took your seat, that was rude of me there is no reason for you to talk to me like this. A simple please move would have been polite." That way nobody knows that you incited everything and by the time you walk away the other person will be stared at and talked about so much the seat will open back up shortly.
lol, hope this helps people. The sticky note idea was funny in the earlier posts, I just had to add a little spice.