I wonder... if you turn the tables on them, if they will change their tune. Seems like someone brought this up in the forums one time...but I doubt I would ever find the thread. Maybe in the future, when someone says (for example), "have you seen that box cutter?"
If it was a woman that asked, you could answer (nicely), "no,sir, I haven't"
If it was a man that asked, you could answer (nicely), "no, ma'am... sorry, I haven't."
Perhaps after you respond enough to them with the "wrong" pronouns, they might get the picture.