We had the same problem

I used to have about 25 beach towels (they were NOT cheap, as you know well) and now I have TWO left. We were going broke providing everything for everyone, my stepsons being the biggest culprits. They are all in their 20's now, and all 3 of them would bring a few friends, and next thing you know they're on the cell phones inviting MORE friends, ugh

Not ONE of those kids or their friends would think about buying a pack of hot dogs or some charcoal, or even some soda. ZIP. Well, my husband LOVED every minute of it and wouldn't stop it, but I put an end to it. I told them that from this day forward, they would have to provide all their party supplies and would only be allowed one friend each, AND they had to call the day before if they wanted to use our pool/hot tub. That was the end of that, about 2 years now

I also before I laid the law down, took my remaining beach towels and hid them in my bedroom closet and guard them viciously

I have since bought a few more for DH, DD, and I to use, but when guests come over and they start looking for a towel, I kindly tell them sorry, we only have enough for us and WHY didn't you bring one?
I swear, the older I get, the less I give a rat's patootie if I hurt anyone's feelings if they are rude. Maybe that's why I have wayyy less friends now?

Oh well, the ones I have left are true friends and behave as such.
I'm so sorry that everyone is taking advantage of you, but YOU are the only one that can put a stop to it. You might hurt some feelings, and you might end up with less friends, but those that don't understand and drop off weren't really friend material anyway. Good luck!!!