Depends on the floor. In my experience you can remove the floor and have the poison fall one level. Not always useful. It has a tendency to get on the person removing the floor, however. Sometimes you can put a floor over a natural floor and have it go away. But if they walk through it on the way to put on the new floor (and by they I mean anything that moves, especially cats) they'll just spread it to the new floor on the way back.
If you're underground seriously consider collapsing the layers above onto the effected layers. Flooding the area with lava may work as well (never tried), flooding with water just pushes it around (did try).
Ultimately, the only way to really deal with a messy forgotten beast at the moment (dust, slime, etc...) is barricade yourself into your fort, kill it before it has a chance to spread it's nastiness and somehow contain the corpse from prying cats, or (and this is usually what you have to do if they get into your fort) restart. I say that last part because there's no remedy for FB toxins, they frequently get everywhere due to cat movement, and they frequently are deadly, regardless of their description, effectively meaning you end up in a situation where either your dwarves will all die before the tantrum spirals start or they'll all die in the tantrums spirals. Either way you've lost.
I typically dig down in shafts designed for that purpose (they have a collapse trap at the top, effectively sealing the shaft and killing any forgotten beast). Once I've found the underground area I cave in parts of the ceiling to create walls at the surface in the caves, then fill in the walls above till they meet the ceiling. That cuts off FB access to the fort and gives me access to a section of the underground. So far it's the only thing that seems to work long term.
-DB