After getting sick of my friends stealing my maps, I decided to spend hours on finding out how to mark maps the right way. There are many different ways you can mark your maps with items that take time to delete. For example, on Foundry you can use a machine gun turret to get outside the level then spawn some item (lets just use a crate) and put it next to the windows. Of course many people know how to do that so they can go delete that. A another way is to put items on the rafters of the level. You can not delete these objects unless you get up to the rafters. Another way is to spawn a grav lift outside the map and put it where the Guardians roam. If you do it right it will have the little blue air from the grav lift showing in the level but you will not be able to get to it. Now for my maps, I like to mark them with reciever nodes. So on Foundry, I used a machine gun turret to get past the fans, and spawned a reciever node outside the map and wedged it into the fence. The node is impossible to delete because as soon as you go past the fence you die. When I post that map, I will put in my sig or in the description the location of the node and telling others that is my mark. You can use these techniques on any level to mark your map. Some maps are harder to mark like sandtrap but who knows maybe you can find how to do it. Hopefully, this helps some people.
You could avoid getting on line when you forage... or change your game party to invite only and don't invite?