I have always bitten young puppies ears when they have 'played' too hard, having watched their mums do it. Not dreadfully hard, just enough to cause a startled yelp. It's always worked to solve that particular issue. But I think that's more about teaching puppies what level of mouth pressure is funny and what isn't.
I don't know how much it'd work on this particular issue.
My ex, who is a police dog trainer always says that any aversion training should be done extremely hard the first time and you may never need to do it again, certainly not much. He says most people's way of try gently, try a bit harder and so on just desensitises a dog to the punishment/aversion. And you make an almighty loud noise and fuss too.
So one way to go,
immediately a dog does something wrong (timing is all) is to
throw a choke chain (It hurts when it hits but not terribly so but makes a clatter too) and go bananas - waving your arms around, chasing the dog, shouting loudly but with the lowest voice you can manage - you should look like Basil Fawlty
A border collie will be pretty traumatised by it all - but it will remember. It completely stopped my dog collie even
looking at sheep, in the days he was a fell-walking pet - he was totally trustworthy.
Now of course, he's a proper sheep dog and they're fair game