Clearing a building of zombies is a matter of engineering skill. You have to stop all possible routes of spawning as well as entry. I've managed to do it but it's not easy, you gotta be smart about it.

On a different note, Yield, can we maybe get some of the new items that came out in 1.4? As in carrots and potatoes, in particular, and maybe nether stars as a super mega rare drop or something?