Hmm, the delete button should be working.  Please make sure you have the latest update (ver 1.50) of the game.   Also, if you are running in Vista, please make sure you have proper permissions for deleting files.

You can completely delete candidates by deleting their file from the following folders:

/my documents/my games/polmachine2008/candidates

/my documents/my games/polmachine2008/candidates/configs

/my documents/my games/polmachine2008/candidates/packages  (this folder is only in ver 1.50)

The file will have the same name as the candidate you want to delete.