What are Wisconsin laws like regarding "no weapons" signs?
Do they have any legal bearing or is it just a "trespassing if caught and refuse to leave" situation?
A combination.
It's trespassing if caught and you refuse to leave, although it's nothing gun-specific. Same thing as no shirt, no shoes, no service, or walking around muttering obscenities while reeking of anchovy oil... being told/asked to leave, and refusing to do so.
The act of refusing to comply with a properly posted sign, (must be "visible" and at least 5x7" in size, and posted at all entrances open to the public) is a er... "class D forfeiture" (or similar verbiage) which is $1000 dollars. Although probably more like $1300 if you actually go to court, try to fight it, and lose in added court costs.
So that's kind of harsh, OTOH, it's just a
forfeiture, not even a misdemeanor.