Depending on how much lead time I have when I see a chasing dog, I'll yell "BAD DOG! GO HOME!" (Believe it or not, I tried "SIT!" once, and the dog stopped and sat with a puzzled look on its face.)
If that doesn't work, I go right to my can of HALT! spray. I've yet to see a dog continue the chase once they get some of that in the face, but it doesn't cause any permanent harm. Bear spray is too bulky, and goes everywhere in a cloud, which can make you very unpopular if anyone is riding behind you.
There was a three legged Chow on one of our regular routes that evidently didn't know it was handicapped. Man, that dog was fast! It never bit anyone, but it would run out in front of the group and on several occasions almost caused a huge pileup. After a couple of times getting zapped with HALT, it would just sit in the yard and bark.