Small dog syndrome/prolific ball thief help me!

[givemestrength38]

I have a mini heinz. Think toy breed but scruffy. Her energy stuns me she has two 1-2 hour walks a day. She is 90% lovely but has a big vice I need help with!
She loves other dogs, has a good recall but is ball obsessed. If i take her own ball out she has zero interest in other dogs but if they come near her or her ball she sound possesed. She also steals other dogs balls in a very sneaky manner she will give up in exchange for food but then thats rewarding negative behaviour!
Yesterday she went to say hi to a dog I didnt notice the ball till she'd stolen it. The dog went for her bit her the works (thankfully not to badly). This has not even dettered her ball obsession and she has no fear of any dog despite them being 20 times her size!
I have kept her on lead since this but would rather find a soloution than walk her on lead forever more. Any ideas?

No ideas at all, but sounds a right character. May I demand a pic?

Here she is looking guilty after appaling behaviour again!

awwwwwwwww

My chihuahua cross Jrt is also ball obsessed if he has a ball he will growl at the other family dogs till they back of and they are all bigger than him but he doesn't care he will get nasty if they try to take his ball, thankfully he doesn't steal other dogs balls.
My solution is for him to not have any balls when he's around other dogs his little tennis balls are just for his one on one play time with me and that seems to work.
I think terriers in particular just love to chase balls.
You might just have to keep a close eye on her when shed off lead and as soon as you see another dog recall and put on lead that way she can't steal any balls
Good luck.