Actually it does. The rules governing impeachment would still apply. It would not be a law enforcement issue until he was removed by congress, because he would still enjoy presidential immunity until that point.
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What part of the Constitution states how a usurper is to be removed?
You're getting hung up on that word. Like it or not, after being certified by congress and sworn in, he's the president. He'll be the president until he either dies, is impeached, or his term expires. There is no need to look for another constitutional means of removal (and there isn't one), since impeachment is still very much viable for this situation.