Lots of things ;
Yell "Get off the couch', 'No!', 'Go home'.
Swerve toward the dog, you then become the agressor.
Get off the bike, keep bike between you and the dog.
My wife carries a (shrill) whistle on a lanyard around her neck on our tandem rides . . . seems to work most of the time.
Some folks use pepper spray, amonia in squirt gun, fire blanks (but all these are a bit risky).