You should be able to safely wash most sound finishes with a pH neutral
cleaner as discussed in the cleaning article in the Floored News section of
this site. I'm sure you have tried washing the stain out of the wood, but by
now it has discolored the wood itself, below the floor finish.
You will need to start by trying to sand them out. Sanding with 80 grit
sandpaper will remove all the finish and some wood. If you are really lucky
the stain will dull and disappear as you sand down through some depth of
wood. But don't go as far as making a gouge in the wood floor, there are better
ways to remove this stain, if it is deep in the wood.
You can certainly try bleaching with the strongest wood bleach available.
You will need to remove all the wood floor finish off just this stained area, by
chemical stripping or sanding. Try a wood bleach called Klean Strip at
http://www.kleansrtip.com. It will turn the dark areas an off white color.
All these treatments contain water so the floor will need to dry overnight
and then be sanded with 100-120 grit to smooth the wood but be careful not
to sand out the bleached effect.
The off white colored spots will need a light colored wood stain to blend
them in to the rest of the floor. It's quite an art. You will want the color
to blend in with the final finished color of the floor, not the bare wood.
Then touch up with the same finish the floor is coated with. If you don't
know what that finish is, read the second part of the floor maintenance
article in the Floored News section.
And lastly, major repairs, or replacements. Or a combination of all of these
methods. Find an old (or young) floor master in your area, that does these
things, and will give you advise in person. But you certainly can do the
bleaching yourself. You can also read my repair article in the Hardwood
Authority section, to see if you can do the repair yourself.
As always your Most humble servant, Joseph, the Wood Floor Doctor.