Kill Switch

Oracle Text

2, T: Tap all other artifacts. They don't untap during their controllers' untap steps for as long as Kill Switch remains tapped.

Card Rulings

10/4/2004 Note that you can’t choose to leave this card tapped during your untap step (unless some other effect says so).
10/4/2004 Only the artifacts that it tried to tap when the ability resolved are prevented from untapping. Artifacts that enter the battlefield after that or permanents that become artifacts after that are not affected.