Is it possible to lock a ceiling in position?
Yes, you need to select the ceiling boundary:
To do this, hover your cursor over the edge of the ceiling or wall. Using the TAB key will help you cycle through to select the wall.
Now select the ceiling select. The Ribbon will turn green and switch to the Modify|Ceiling tab.
Click Modify|Ceiling tab > Modify > Pin. This will now locked the ceiling in place.
If you need to unlock the ceiling select the ceiling and unpin either from the ribbon or on screen a pin will show in the middle of the ceiling.