be (as) dead as a doornail
1. To be no longer alive, for certain. (Doornails were hammered in a such a way that they could not be reused.) I poked that squirrel with a stick and, yeah, it's as dead as a doornail.
2. To be totally defunct. Oh, that idea from last week's meeting is dead as a doornail now that the CEO has vetoed it.