Scrape off what u can with a razor blade. White vinegar should remove what's left.
Clean this off with water and a cloth rag or a brush.
soap, water, scrub
Acid Water
BACK-SET by RoMix states it will do the job.
I use one of those straight razor holders and scrape it off.
A good degreaser should work. Also, look into muriatic acid, that is supposed to be a good concrete cleaner.
Check with your local hardware store or Home Depot/Lowes etc. There is a special chemical that is supposed to clean everything from concrete.
Mixture of 50/50 methylated spirits and water
the best way to clean you concrete floors is with hot and soapy water and lots of hard work
There are a few ways one can remove wood stain off of concrete. One way is to use a sandblaster to remove the stains. Another method is using wood bleach, water, and a scrubbing brush.
Clean
We would need to know what caused the stain and what kind of surface it's on. The procedure to clean an oil stain off of concrete is going to be much different than cleaning a red wine stain off of furniture.