"Final Cut" after shuffling an opponent's deck
The Comp Rules were changed a few years ago, to no longer allow the “final cut”.
The old wording allowed you to cut your own deck, but only cut it, and only if your opponent actually shuffled it. (You couldn't do a cut if they only did a cut.)
However, it's much more likely that you'll simply instruct a player who does that of their error, and tell them not to do that again. “Deck Manipulation” is no longer described in the IPG at all, and certainly not as a Game Play Error; it was a sub-category of Cheating (Manipulation of Game Materials, more precisely). Since it's not a specific infraction, and it's a violation of the MTR (not the CR), it's one of those “don't do that again!” things.
Unless, of course, it becomes apparent that the player knew they can't cut after the opponent shuffles, but did so anyway because they thought it was to their advantage to break that rule. That's still Cheating.
Short version: not allowed; please educate.