And even if you give them decorator privilege, nothing can be stolen. If they hide it, you can retrieve it and place it again, from the contents list, without tracking it down.
Only the owner can add or remove furnishings. Other players can be given decorator permission if you choose to do so, and then they can move furnishings around, but that's all. They can't even take them through doors because they'd have to put it down to interact with the door.
The absolute worst someone else could do to your home is something like sink all the furnishings into the floor so it looks like they're gone and you have to use the menu to retrieve them, or arrange them into some sort of inappropriate shape.
If you want to prevent that all you need to do is set it so they have visitor permission but not decorator. That means they can come to your house whenever they like, but they can't change anything.