It doesn't come off with water, needs some chemical attack.
I've used mild lye solution. First, spray water on the area.
Apply lye solution (1/4 tsp of lye into a cup of water) with a brush,
wait a few minutes, rinse (water spray again) and mop dry.
Rinse out your cloth and the brush, or lye will attack it. And
use gloves, of course.
I don't know what the stuff is, but the lye treatment seems to only
be necessary a couple of times a year (soapy water is the usual
cleaning medium). My seats are painted, so the paint will soften
if I overdo the lye (but plastic is not affected).
Ick. Why not just buy new seats? For the cost of what hasn't worked so
far, you could probably replace the seats.
With wooden ones.
You could try white vinegar to clean it. Put it on neat with a cloth
and leave it, then wash off with hottish water. You should clean your
toilet and seat daily to keep it clean!!!
I raised two sons, so I could probably be paid as an expert witness. :-)
Warmest Regards,
Donna