Gold Kryptonite
A character holding this object can't use any powers except Super Strength

So, if I Sargoned out a character with Batman Ally holding the object, I shut off (i.e. he can't use, not that I've countered) his Stealth too, correct? Along with Willpower from any indomitable/utility belt

Correct.

/filler

Right. If you can get an opposing character to hold that Kryptonite then they can't use any powers, including those they would normally be able to use through traits, team or combat abilities: whatever. And since it's not countering or ignoring things that say "can't be countered/ignored" is still reachable.