Let's say I Mind Control a figure now, I know I can turn off there impervious and it stays off. So, is mind control the only way to cancel Impervious on a character?

Karma MCs Vet Supes Supes turns off Impervious does it stay off for the rest of my turn?
Does that turning off of impervious count as a MC action?

Didn't you just say that you know you can do this?

Just in case I misunderstood the first sentence of this post, yes you can use MC to turn off an optional power.

No. Turning off an optional power is not any kind of action; it just happens.

Could someone provide a list of the optional powers that can be turned off in this way? Also, how long do they stay turned off? Till the end of your (the mind controller's) turn?

Check the PAC. If it says "optional" next to the power... well, there you go.

From the Icons rule book (page 13, under 'Powers'):

Powers are in effect when they appear in the stat slot. If a power is optional, it is assumed to be in effect during an action unless it is canceled at the beginning of that action. When a power is canceled, it is canceled until the end of that turn and resumes effect at the beginning of the next player’s turn.

Perfect, thanks very much