Focussed protest campaigns against selected mercahnts banning gun carry
The strategy to deal with this is to to open carry discreetly as you shop and NEVER ask permission.
This forces the merchant to either enforce with direct orders to the open carrier to leave the premises or prominently post (posting unfortunately has been hard coded into Wisconsin law such that entry with reasonable or actual knowledge of the gun ban is apparently unlawful - in most states, gun carriers can ignore gun buster signs without any criminal penalty, not even "tresspassing").
Once the merchant takes a stand and provides notice, THAT MERCHANT needs to be put on notice that his establishment is going to be boycotted, and to expect negative publicity, including possibly open carrying protesters on the streets and sidewalks in front of his establishment.
As the weather warms up this year, select some of the smaller establishments for focussed attention by gun rights community - they can be great weekend family fun. Action in the public eye against such merchants will sometimes result in policy changes and/or quiet sign removals. But more importantly, even if the targeted merchant does not change policy, other merchants will be detrred from posting in the first place, and/or quietly remove their gun buster signs.
Remember, we have a lot of consumer power here - merchants do NOT want to post, and do NOT want to order their employees to confront open carriers and publicly create a scene by ordering them to leave, and they surely d NOT want picketers in front of their store fronts.
Call out one of these bully stores at a time and focus - taking them all on at once in a broad boycott campaign is fine but it's the intense focus on one of these bullies which will get you publicity and high payoff for your actions.