Okay, that's the information I was looking for. It's still impossible to know for sure without looking at the dogs in person, but I'm hoping I can give you some thoughts to mull over on this issue.
Herding/working dogs tend to have two main reasons for looking "soft" on corrections.
One, they are very independent and have a low tolerance for human meddling. This is very common in Aussies and many breeds that go back to the same stock that Aussies do - catahoulas, ACDs, English shepherds, etc. These dogs want to know what the job is and then be left to figure out the best way to do it, on their own. The "fine tuning" you can do with other dogs makes their heads explode. For these dogs, it's best to set things up so that they think the right way, is their idea. Also, you can increase their tolerance for meddling by doing it in such a way that they don't get "taken out of the work."
This is awfully hard to explain, but you can do it with things they like to do, asking for really easy additional behaviors in such a way that it's part of the "job", then things that are a little bit harder, etc. A dog that likes Frisbee you can ask for a recall when they are looking for the disc and the SECOND the dog responds, throw the disc. Then make it down. Then make it come around you, and so on. Keep it moving and easy, and you can correct ("Ah-ah";) but don't ALSO hide the disc like you'd be tempted to do - that's a double correction. Avoid those at all cost. Don't step towards the dog while you correct - also a double correction.
On sheep, you don't block such a dog to force them to change direction, but use the sheep to draw the dog around - step through the sheep at the dog's tail and back up quickly. The dog must then change direction or crash into the sheep. If the dog chooses "crash into the sheep" as an option, then you've got something a bit more obvious to correct! A bit more on making corrections black and white in the next part on "worriers."
You might be thinking, "What the heck does this have to do with being able to stop my dog from posturing at other dogs?" This builds a relationship where the dog trusts your correction and actually enjoys it, having long associations with "work."
Okay, so the other possibility is that you have one or more worriers in your pack. You can combine a worrier with a task-oriented dog and get a dog that really, really hates interference. My Ted is a worrier but is a team player, and the team player part is stronger than his worry. However, if I am not clear in what I'm asking, his tension rises and the worry takes over, and he wavers between giving up and simply taking matters into his own hands (probably the "playing" you mentioned). Worriers notice everything and their fear comes from the expectation that they are supposed to do something about it. Worriers are just a step below fear aggression, and a worrier pup can become that quite easily.
How you deal with this type of dog is easy to explain but will take a lot of thought and honest self-evaluation to implement. Very simply, a worrier needs things to be very clear. The boundaries need to be clear, what you are asking needs to be extremely straightforward, and it helps to have a point to a task, rather than being training for training's sake. A correction blows up their world - most of the time it's too much information, at the wrong time.
It would be as if you were a construction worker and you riveted an entire beam the wrong way right in front of your boss, and then the next piece didn't fit and the boss started yelling at you THEN. Some people can blow off this kind of correction, maybe even get a laugh out of it ("The boss fell asleep at the wheel there!";). But some people are thinking the whole time they are working about every detail, maybe even taking some pride in it how well they are doing, and when the boss yells all they can think is, "Where did I go wrong? Do I suck at this? Maybe I should quit!"
In the round pen, you'll never ask this kind of dog to circle, circle, circle - rather you'll want to get this dog doing figure eights, taking sheep off the fence, downing/stopping when asked, etc. This dog may seem very disobedient and keen but then take you by surprise when you open your mouth to correct something they do, and they shut down completely! This is the dog that wavers between eating sheep for lunch and looking for the exit.
Off of sheep, you can increase the confidence of such a dog by simplifying your requirements at first. If you don't want your dog to counter surf, ask him to stay with you, in sight, at all times. Don't leave such a dog loose in the house until they've completely matured (and we're talking years from now, if then). This works for the task-oriented dog also - "your job is to stay by me." They understand that easily - it's more difficult, as Anne pointed out, to understand the difference between "okay because I'm not around" and "not okay because I'm around" - that's an extremely mixed message. You want the message to be: "It's okay because I'm around and I'm not saying anything." Then they have the confidence to experiment and know the world won't fall apart just because you have something to say about their actions.
I hope this makes sense.