One thing this thread has taught me: Gun owners are poor readers.
If you own private property and do not want gun owners to carry on your property, you need to have huge international language signs consisting of a red circle with a line across an image of a handgun. Circle needs to be 18" to 24" across. You probably should place it in multiple locations near the entrance to the building. Inside, it would be wise to install a metal detector.
Otherwise... well, you are on your own.