>> If someone has stock in a chametzdig corporation,
I see quotes By Dayan Shlomo Cohen from Igrot Moshe Even Ha’Ezer 1:7 that an insignificant ownership is not a problem (Rav Cohen’s estimate 3%), and Minhat Yitzhak, Vol. 3 1:11 that it is but be sold with your other hometz. The latter opinion makes investment in treif, shabbat violations, forbidden interest. Badatz offers kosher stocks.
On a lighter nbote, I hope you do not hold HMZ and HMTZ stocks (looks like some Metals and a Russian Chemical plant, but they just do not sound right!)