Yelling and stopping works about 90% of the time for me, although I hate stopping.

For the other 10%, maybe in conjunction with stopping, Halt! See if you can find the plastic clip that holds it on your handlebars. Make sure the nozzle is pointed away from you, not as a safety issue, but because when you hear the clicking sound of dog toenails 15 feet behind you and closing fast, you don't want to have to mess around figuring out which end is which. Grab it, wait for the beast to get in range, and spray. Practice in a calm area ahead of time, it's pretty easy to aim well enough and you'd have to work hard at it to spray yourself. It doesn't matter if the plastic clip stays on the bars or the can, you can get it back together later.

For a pack, hit the closest one. The rest will learn quickly.