No idea what "spank" is. Is it a Move? Maybe show a cropped screenshot (alt+shift+s to select the area to screenshot, then paste it here in a reply).
I generally found the game well explained through the in-game tutorial. Of course, some limited familiarity with the game it's a parody of (Darkest Dungeon) probably helped me.
Okay, I'm pretty sure this is explained in the tutorial. When an adventurer's lust gauge reaches maximum, she can receive one of various afflictions, depending on various factors. When she's has such an affliction, they have a chance to be limited to some special moves. To get rid of the affliction, you need to lower the lust gauge again. All of the limited moves will lower the lust gauge (and probably inflict some kind of negative effect), but you can also make them masturbate, which has no negative effect.
It's preferable to instead lower the lust gauge before they receive an affliction. You can do that by packing anaphrodisiacs as part of your rations, or leave adventurers with high lust in your base while also staffing the tavern.
Although the "sucking off the enemy" part is probably a grapple action by the enemy. Rightclick them to see their bestiary entry, which includes their moves. I'd recommend to read that for every new enemy you encounter.
