I know it used to be, but now powers aren't 'optional' any more. So if your opponent rolls doubles and you have Charge can you still choose to take the knock back?

Nope. For better or worse, you're staying in that square.

Yeah dude...there's no way da BLOOOOOB can chose to not be knocked back! I mean..."Nothing moves da BLOOOOOOOB!" :P

You never could, unless you canceled your Charge before getting hit. Now, though, it's impossible for a Charge fig to get knocked back...unless they've got Combat Reflexes, maybe?

That's a great question: Can a character with charge AND combat reflexes choose to be knocked back?

From the PG:

 A character that is normally immune to knock back that has Combat Reflexes can choose to take knock back when hit with a successful attack. When this option is selected, the attacking player still gets to choose the direction of the knock back (if a choice can be made).

However there is no longer a choice in the direction.