A better question is how *high* do the boundaries go? It's been a while since I've cracked the FAR's, but I think that regulation of flight activities is a federal activity, not a municipal activity. While the FAA will work with locals for traffic planning (ie, publishing preferred traffic patterns, noise abatement), I don't know that it can ban this specific type of flying activity over a particular city.
Nor a State activity, for that matter. In California, the State has ceded authority to charter cities to use their zoning powers to regulate landing areas, ie ground-based support structures, but unless the federal law has been changed in the past 24 months, they have nothing enforceable to say about overfight activities.
So, for instance, you won't find more than a half dozen private-use helistops in the city of Los Angeles, and will guarantee yourself a visit from the LAFD and Building divisions if you land your chopper on your building's emergency helistop, but an extremely anti-social nut could hover over the Hollywood bowl during the 1812 overture and the city would be powerless to stop that.
Maybe rotary-wing aircraft are special, but I don't think so. (If so, I suppose you could hire someone to tow your sign with a helo ....)
IANAL, but this ordinance looks like a local jurisdiction thumbing its nose at the rule of law, and passing a 'feel good' measure to satisfiy a local political constituency. And that stinks.
Can the city ban overflying French commercial "SPOT" imaging satellites from taking pictures of private and city property and selling them internationally for profit?