Well I don't think it would be possible with what your showing. What version of classifieds you have. See that query is only in one spot in the program on line 159 of showcat.php
Code:
Content visible to verified customers only.
You error states the $cat variable is this last14?ABCDEFGH and the query there will only be executed if cat is a numeric variable and it is not in your example
I will do some more testing but looking at the code and the example since it is not a numeric variable that query would never be run.