To prevent a piece from being accidentally modified, you can block it. To do that, you need to select the piece and click on the Block/ Unblock button. As long as the piece is blocked, it cannot be modified, moved, deleted.
Select the piece/pieces you want to block.
After applying the function a mask will be inserted over the selected piece/pieces so that it will be easy to recognize the blocked pieces.
If you wish to unblock the piece, select it and click on Block one more time, at which point the piece can be modified, again.