You can find the most stop signs in Pleasant Park. They are located at the junctions. Just bash them with your pickaxe to complete the challenge. You can destroy the same signs in different matches so you can come back if you did not destroy enough stop signs in the destroy 10 stop signs challenge.
First select the Catalyst outfit in your locker, this is required to do the challenge. If you do not have this outfit equipped it will not count for your challenge.
When you have the Catalyst outfit just go to Pleasant Park and destroy 10 stop signs with your pick axe. The signs are located on each junction. You can come back next match and destroy the same signs!
Destroy stop signs video
In this short video we show you where and how to destroy the stop signs.