Hi Sara The best way I have found of removing stains from concrete is acid etching. Get a gallon of hydrochloric acid (muriatic acid) from a swimming pool accessories shop, dilute with 1 part water to 1 part acid (add the acid to the water), pour over the stain, let set and then hose off. Scrubbing with a bristle brush will help. But be prepared. If the concrete has aged and darkened, this will etch it back to a lighter color.
Might also try TSP (trisodium Phosphate). Make up a solution, put over the stain, let set and scrub off......but I haven't had too much success this way.