I thought I posted, but seem to not have hit that button...
I wanted to mention that our local RSPCA has a course called "Recall clinic". I don't know if only this one does it or if RSPCAs in other places would have something similar.
My old dog used to try and block dogs from going back to their owners by using threatening body language if they dared to move. Very naughty. I managed to usually call her back in those situation, but sometimes I had to go grab her and give her a taste of her own medicine.
If you see the other dog is trying to block yours, it's best not to call your dog but just go over there and lead her away, whilst keeping an eye on the other dog's moves.