I had this issue on a RSS feed recently. I fixed by doing a string replace on the offending characters in the php code, but somehow I doubt that's an option for you (or probably the best way to do it either).

I'm guessing ’ = ' though I've noticed some editors use ’ for apostrophies (its slightly slanted compared to the '). I'm not too sure the effect of one over the other, but just try using ' or ’ in your posts and see what happens. (copy paste from this post since hitting the apostrophie key will just repeat the original symbol in your post editor).

Somehow though, unless you have access to the code or database, I doubt you'll be able to fix this without editting all your posts. Though I may be wrong, I'm still quite new for stuff like this...