It's simple. You entrap them, give them the classic surrounding visual and audio stimulus for breaking eager heroes into bimbos, drones, or maids. And give them a very special brainwashing track. "You are a stalwart hero, you will resist the mental changes! Every one of these weasely suggestions to resist us, we both know that you're strong enough to resist! Be good and resist!"
Now, they're brought to immediate confusion. Why would their enemies, known for mind altering all of a sudden simply say that one should not to fall?
It must be a dirty trick. Can't listen to this vile evildoers drivel!
Closing their eyes and covering their ears over.
And... Silence. Nothing strongly lit enough to reach beyond the eyelids. Nothing mind affecting getting through to the hero, left alone in their own processing thoughts. Maybe even time enough to consider an escape plan.
But wait, would there be harm in trying to break myself out? Surely there would be retribution. And waiting would be harmless, all I need to do is resist.
Wait. Surely that's what they want me to do, to give them the extra time if not to break me, then to break my would-be-saviour friends!
Resolved to escape, carelessly dropping their arms down and opening their eyes back to the brainwashing room and being told praises, "a good idea and a good job to keep our words from reaching you! you have to resist, hero!" - continued encouragement to resist as they scramble around in the impassable cage.
Only after thorough exploration of the walls and finding nothing will the audio track evolve to have tiny little resists ceaselessly echoing quietly, subtly enough one could be unaware of the extra pestering noise.
Continue until they inevitably collapse from exhaustion. Let them stew in resistance.
And when they awake, immediately ask DO YOU KNOW WHAT RESISTANCE MEANS?
Resistance is fighting against influence, resistance is staying true to yourself, resistance is losing the fight, resistance is giving in so thoroughly you can't even feel it happening, and you've resisting this entire time, being the same hero you got trapped in here as, a great job resisting us.
You should brag about how you have done nothing but resist when your friends come to rescue you!
And they will come, your resistance has given them so much time to plan a rescue.
They'll just be paranoid about you, knowing who captured you. So comfort them that you've been good and resisted everything. Now, what does resistance mean?