According to actual studies, Coke douches are an ineffective form of contraception. You are likely not surprised at this.
However, bitter lemon (a soft drink with quinine and lemon) is a fairly effective contraceptive douche. (Look near the bottom of the page, for an excerpt of the study, and the citation.)
(Uh, obviously don't rely on this or anything. Spermacidal post-coital douching isn't going to be incredibly effective anyway, because there are already going to be thousands of sperm in your uterus by that point.)