I'm sorry to giggle Simon but I think she's a bit of a smart a**e...
I don't know what else you can try except playing fetch on-lead so you can re-inforce your command to come back, or ending the game when she runs to someone else. When strangers give in and throw for her she is getting exactly the reward she is after - their attention.
Naughty girl. But so cheeky!!!
Reminds me of a collie I babysat once for a couple of weeks. If you told him off, no matter how nicely, he would look at you like you were just despicable. Then he would take off - and escape! The first people he found would always fawn all over him - "look at the beautiful Lassie dog!!" and he would look at me as if to say "See, someone in this world appreciates me!" So he always got rewarded for his actions.