Don't look the dog in the eyes, but yell "Go Home" and/or "No" forcefully. Don't increase or decrease speed and be aware that vehicular traffic may be cluless to what is going on so will not be ready for you to swerve. Have Mace, Halt or similar easily accessible on your handle bars and do not be afraid to use it. It will only have a temporary physical impact on the dog, but may help it learn that cyclists are not lunch on wheels.