Greifers

So i built a house, with one tall stairway enterence (shame on me i suppose) and last night while I was logged off, someone came in a blew out my metal door at the bottom, and then the door at the top. they didnt really get anything, (all junk really) however they broke the stairway i had and left a wooden ceiling in the room basically locking my ass up like rapunzel in my sky fortress.

If i am not mistaken, ceilings can only get taken out by decomposition, right? I cant blow it with c4? So unless i drop out the window and make another way up, im SOL i take it.

Verification on this would be helpful, also if they made it where people who didnt build that foundation couldnt place anything in that house, That would be greaaat