Some lost soul has pleasure to damage people land.
To prevent hours of work be ruined by those idiots please follow those steps to protect it:
Go to World > Parcel Details
Select the Options tab and make sure Build & Object Entry is unticked for Everyone:
If you have multiple region you check all land that are unprotected with the following MySQL query:
SELECT name, landflags FROM opensim.land where ((landflags & 64) or (landflags & 16) );
When you place an object some time it can be moved by Anyone
Go to Build
Use the Edit tool to select all the objects on your land and make sure none are set to Move:
Here is a list of confirmed Griefer UUI.
Feel free to add it to your ban list.