brazz,
I heard that alcohols and harsh stuff like bleach will craze the acrylic and ruin it's appearance. You might try drugstore grade hydrogen peroxide as that has gentle bleaching properties (and is non-toxic.) Spot-test it first. I think you would have to leave a soaked pipe cleaner in the stem overnight, at least. Also, keep it in the dark since H2O2 degrades in ordinary light. That's my 2¢ worth. Good luck.