Hi meat:
Although Dexter is obviously a happy-go-lucky typical labrador, I'm sorry to hear about your trials and tribulations when out and about.
I have two collies and I also on occasions take a ball and I too use a ball thrower. My younger one is quite obsessed with the ball and he also used to try to chase other balls belonging to other dogs. He doesn't do it now but I had to be quite cruel to be kind.
I eventually broke him of the habit by literally screaming at the top of my voice at him, getting literally right in his face as I did so. It had to be done and although on one hand I felt bad about it, and quite embarrassed in front of the other dog owners, he would still have been doing it had I not. It took a few "goes" but he soon got the message and is easily dissuaded nowadays with just a gentle but persuasive "no!"
Also, I don't know if you could possibly find somewhere to exercise him where there may be few other people with dogs?
Plus, when he drops the ball at your feet, order him to "sit" and "stay" a few feet away before trying to pick up the ball, and don't do it until you're satisfied that you can get to it before he can! He must learn that you are his boss, not the other way round.
As suggested, the halti is a good way of controlling his pulling because I know how big and powerful some labs can be. So don't fight him, control him!
Hope this helps. Best of luck.