When out with my pet Border Collie called Max who is 3 I have a problem with him,if he finds a ball,he'll run off with it and he just won't come back to me even with treats,eventually I will manage to get him,if anyone has any ideas on how to cure this,please leave your comments.
They do get obsessed with toys don't they. I'd suggest a bit of intense training over a short distance with lots of fuss and praise when he brings it back. This should reinforce that he will get 'his' toy back every time.
This probably wouldn't work for most dogs but my Springer used to do that and I'd just sit on the ground. The trying to get it back is no longer a game.
Keep another ball in your pocket and when he runs off with a ball he has found, or one belonging to another dog, just start playing with yours so he can see you. He should come running back then to join in the fun.
I hate it when another dog runs of with my mutt's ball. Sometimes it takes the owner forever to get it back to us. Luckily my dog will drop any ball he finds at my feet for me to throw it for him.