There isn't a comfy solution for this. You're basically going to have to be the bad guy.
Tell him that the bag stuff costs money and you didn't plan to buy for two people. If he wants to chip in for a community supply, you might agree to do that but otherwise he needs to get his own stuff.
As for the balls, I'd be very direct about that. Inform him that no one, not even your own Mom or Dad, throws your stuff without your prior OK.
If this causes a problem, remove him from the team (if he's your teammate). If he's some random guy from another team, my first order of business after explaining your policy to him is to find your sergeant-at-arms and tell him you're going to be needing his official duties later that night.
Jess