Go on YouTube and watch a few videos on redirection training.
Also, does your dog have a favorite activity? My GSD loves balls, so if I need to redirect her I can pull out a ball and she will focus on that. My dog is trained to sit and stay, so I will take out the ball, make sure she sees it, then I have her sit down or lay down. Only after she is in a down will I give her the ball.
Dogs don’t need to play with other dogs, they really want to be with their people. They can be very enriched by playing with you and interacting with you in a positive manner. So if you take your BC to the dog park, plan to do activities with them that they love. They will.want to do that activity and will eventually ignore the other dogs.