i had this happen before i tried EVERY cleaner known to man. from babes, to powder pot and rust cleaner, oxy, dawn, there were like 4-5 more. nothing would touch it. finnally had to use bleach. after re-apply for almost an hour keeping it wet with fresh bleach every 5 minutes (used a cotton ball) it completely disappeared.
luckily it was on a white area.
once it was out, i rinsed it good, wiped dry and apllied babes seat conditioner to restore the moisture and protect it and keep it soft