You have my sympathy - Maggie is a master ball thief, even to the point where she goes up to dogs at the park and stares at them till they drop their balls... and one dog seeks Maggie out and brings her the ball I can get the ball back with a treat - but its a double edged sword as she now sees that as a reward for getting any ball and bringing it to me.
I used to take my own ball with me and use that as a distraction for her, but she's not supposed to chase balls and she gets so hyper if she knows I have one thats not being thrown its counter productive...
So my tactic most days is try to guide her away from the dogs with balls and kit myself out with treats for when she does pinch one...
Tilly is exactly the same right down to the not being supposed to chase balls thing. She's a terrible ball thief and will stalk people with chuckits and dogs with tennis balls to steal them, I have in the past had to literally drag her away from an active chuckit
I've got the voice of doom 'LEAVE IT!!' down to a fine art now and if I say it in time 99% of the time she'll come away - undoubtedly while doing the Cocker equivalent of giving me evils
but if I don't see it in time she'll be in there for the kill and then I have to haul the ball out of her gob. Treats mean nothing to her compared to a ball; if I bring a ball for her and let her carry it she stops trying to steal but then she doesn't really enjoy her walk properly as she's fixated on the ball the whole time and I can't throw it for her because of her osteoarthritis. I generally just try to head her off at the pass and either stop her seeing the chuckits/balls or tell her to leave them, I guess luckily if she does nick a ball she will come to me when I call her so I can get it off her again although I've had to take the same one off her 2 or 3 times in a row before
ETA - on the plus side I haven't had to buy a tennis ball in years as Tills manages to find an abandoned one on almost every walk we go on - she's become renowned for it at Hearing Dogs, she can sniff out a tennis ball from a mile off