If a dupe soils their suit and then unequips said suit they can't remove their soiled suit debuff unless they re-equip a suit and use an atmo suit station. I tried building a shower, but that wouldn't remove it either; only building an atmo suit station and checkpoint, delivering their suits, and then forcing them past the checkpoint so they equip/unequip their suits will remove it
Equip atmo suits. Fly in rocket ship to another planet. Wait for dupes to soil their suits. Unequip said suits. Dupes have permanent debuff until you build an atmo suit checkpoint/station, deliver a suit, and have the dupe walk past the checkpoint.