I'm no expert, but perhaps you can show proof of your return flight leaving Mexico at the end of the last visit (assuming you flew) to clear this up?

Perhaps the best thing to do is call the Mexican Embassy, explain the situation with your documentation that you were not in Mexico from the last visit, and see what can be done. I would only imagine that tearing out the page would lead to trouble, as could defacing the stamp, but if you aren't in the system as being rejected then a new passport might do the trick...but then again, do you want to take that chance?