Deleting Masters from Wireframes
Masters can be deleted from wireframes by selecting the master and pressing Delete on your keyboard or selecting Edit->Delete in the main menu. Alternatively, you can Edit->Cut or Ctrl+X masters from a wireframe so they can be pasted later.
If a master is placed in the background or locked, it can be deleted the same way. However, if unlocked widgets are also in the selection, they will be deleted first and the second Delete will delete the master.