Chalk as many as you can. Let them know there's more of us than there are of them. If they clean up one, do a different one a mile away.

Why stop at chalk? Use something more permanent like road paint?

The argument I've heard is that it may reduce the severity of any charges if you're caught doing it because it isn't permanent. It may not remain visible as long since it can be washed away with water, but it still costs the city money to make people go clean it up.