Grundy1133 wrote: business owners can't prohibit people from having guns on their parking lots. It would keep citizens from exercising the MPA which is illegal.
Cite the law please.
I think there is some confusion here on 2 different laws.
MPA gives you the legal right to carry a firearm in your vehicle (if you are legally able to own a gun). That legal right exists independent of the parking lot law.
The parking lot law says that your employer cannot fire you for having a gun in your vehicle as long as you meet certain requirements (concealed, etc). This law came after MPA. Before this law was passed, an employer could fire you if they discovered a gun in your vehicle, but you still wouldn't have violated any laws. The parking lot law has some exemptions for specific types of employers / locations. But that law is not relevant in the OP's case unless there is an employee / employer relationship between the home owner and the people coming to his house. Based on the OP's description it does not sound like that is the case.
So based on this, I believe that the home owner could terminate their business relationship with a visitor / client if they somehow discovered a gun in someone's car. They could also ask that person to leave the property and never return. But no one would be violating any laws regardless of 30.06 (or other) signage. Others on this forum have expressed a different opinion on this last point.