Not sure that feeling that way is sufficient to barr people from entering a public building.
While I think it’s clear the mayor is biased here, I also don’t think a city hall must dedicate space for every group asking for space to do something.
Personally I’d use the rejection to my advantage.
Stand out in front of city hall for petitions, with signs saying the mayor won’t let us in for taking petition signatures.
Besides since you know the guy is against you, he can’t have anyone lurking down there to see what you’re doing, how you’re doing, and pass that info along to other dems.
Also I would have people in front of the Marinette post office(s) with signs and clipboards. Perhaps at their biggest grocery stores.