Is there a way to stop solicitations from being placed on our cars in the church parking lot?

It has been determined that we are solicitation free building so no solicitations are allowed in the building. We are a Campaign Free Zone but that only applies to days in which we are serving as a polling location. While we could try and purchase and place signs around the lot that will not assure those materials will not end up on our cars. We would like to emphasize to the members who are engaged in campaign efforts that you discourage your campaign teams from placing materials on cars or property around our building.