I was playing in a game where I was able to mind control an opponents piece and have that piece turn around and attack his Yukio (with Crossgen ability).

My opponent says that he is turning off Crossgen so his own figure doesn't get backbit. The attack hits, damage is done, and the Mind Conrtrol is over.

Next action on the same turn, I target the Yukio with one of my figures, hit her, and do damage. He says that the Crossgen does its backbite because Team Abilities can be turned off and on by action. I know WildCards can be changed by action, but this was a true TA, not borrowed.

I read the rules that TA's were optional and like optional powers, once deactivated stay deactivated for the remainder of the turn.

Who's right?

My understanding is that once team abilities are turned off, they stay off until the end of the turn.

You are correct. Team Abilities are optional, but once it's turned off, it stays off until the end of the turn.

Just for fun, let's say you mind control one of your opponent's figures and he quickly says he is turning off all optional powers and team abilities before your mind control takes effect. According to the FAQ you still get to use any powers and team abilities you want regardless of what he turned off. The exception being anything that was Outwitted prior to you taking control remains Outwitted while you have control. Any optional power or team ability your opponent turned off before you gained control reverts to off when you give it back, even if you used it. Any optional power or team ability you turn off while controlling your opponent's character remains off once you give the figure back until the end of the turn. Thus you can Mind Control KC Superman, use him for an action and turn off the KC team ability and Impervious. The KC Team ability and Impervious will stay off until the end of your turn.

Mind controllers are good, up until they start going after characters over 200-pts. or a fiqure with (Battle Rage).

MARVEL HEROCLIX fiqure Morgan Le Fey has 3 bolts, but she'd better focus on opponents under 100 points or she takes alot of damage for Minding other folks business !

Remember to turn off any optional powers or a TA before you do the MC action. After the action, the figure immediately returns to its owners control and it is too late to turn off any option powers or a TA.