The following link walks you through all possible cases and is taken from "apple's" website.. I believe that you should be able to delete your unwanted file after following the different steps there..
please let us know, if this solves the problem..
The following web site has some useful Mac utilities. In particular you will find these utilities to force delete or force empty the trash can, namely, Cocktail, Mac Pilot, Panther Cache Cleaner, and Tinker Tools System.
Hope this helps.