Seems rather simple; affected business owners should sue the guy. Minimum relief sought should be the price for full page ads for each affected business in the local newspaper, with the perp apologizing for his vandalism and indicating that he had no relation nor permission from store owners to put up those signs.
And then the local DA should file hate crime charges, explicitly pointing to the apology as proof of the perp’s guilt.
...and indicating that he had no relation nor permission from ...
Is Austin too liberal to charge him with impersonating a city official?
Somebody's got to file a complaint with the bar. The more the merrier, including the city attorney.