Unless someone has a secret they are keeping to themselves, I'm afraid the only real way to get rid of it is good old fashioned scrubbing. You want to use hot water, as hot as you can stand it. Maybe even boil the pony first. Then use the back of a butter knife and scrape it fairly hard against the offending area. Pindot is dirt and makes ponies have blackheads, basically. It gets into the pores of the plastic quite easily. If you take a bit of mud and rub it on a G1, you'll give it pindot.
Anyway, hot water and a firm hand are what's needed. Some people have claimed an Oxi bath can help get rid of some of it. I stopped using Oxi on ponies, so I don't know if it's true.