An anti-union boycott would seem an appropriate response and likely to have a lot more impact.
If I lived in WI, I’d make it a point to visit any business I saw with a sign, and tell them that I was not going to be a customer solely because of that sign.
There are a lot more taxpayers than union members, and it wouldn’t be too difficult to have multiple private sector taxpayers visit a store each day.
I imagine the private taxpayer owning the store would get their priorities straight real quick after a week or so of this sort of “boycott”.
If all the business owners stand together and refuse to be bullied, the union thugs will have nowhere to shop.