I had the same problem. Use toilet bowl cleaner, the kind that squirts upward in the toilet. The Works was good stuff, wife got it at the 99 cent store. Spray it on the spots, when it quits foaming, rinse with the hose. Might take 2-3 times, depending on how bad the stain is.
"When I was younger, I could remember anything, whether it had happened or not." - Mark Twain