 border collie
I used to have a border collie...(great dogs!) they are very smart, but unfortunately, they are learning even when we don't want them to, and they pick up bad habits as quickly and easily as good ones. The good news is, they will do ANYTHING to please you, so if you can spot what's triggering the problem for him, you can usually help them learn the right behavior...the bad news is they can become "obsessive" about their behavior if they aren't given TONS of exercise.
If you haven't had him in an obedience class, I'd recommend it, even if he already knows his stuff, just for the experience of being around other dogs and being required to behave himself and follow your commands. Talk to the instructor about his behavior and they might be able to work with you to help "socialize" him around other dogs, hopefully graduating to an off-leash situation.
There are also lots of places around now where you can take him to learn how to "herd" a few sheep or goats. That might be a better form of off leash exercise for him until he's figured out how you want him to behave around other dogs.
Lastly, you could try keeping him on the leash while at the leash free park to give you the control you need to show him what's allowed and what isn't and let him "earn" his freedom when he behaves appropriately...which might make more sense to a border collie than a time out.
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Border Collie chases dogs, barks in their face 
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