Choose one —
• Destroy target artifact.
• Creatures without flying can't block this turn.
2/9/2017 You choose which mode you’re using as you cast Destructive Tampering. Once this choice is made, it can’t be changed.
2/9/2017 Because the effect of Destructive Tampering’s second mode doesn’t change the characteristics of any permanents, the set of creatures affected by it is constantly updated. Creatures without flying that enter the battlefield later in the turn won’t be able to block.