GreenTools for Google Apps (G4G) has been updated to include a function that can both delete a file and empty the trash.  The example code shows how this is done:

#g4gdrv_setValue('id':'yourid@gmail.com');                                    
#g4gdrv_setValue('file_id':'1Zs4bc3jueqwgw1uLbyo7C5_k_I5pXRfl');        

rc = #g4gdrv_deleteFile(errorMsg);  

The example above will delete a single file from your Google Drive.  If you wish to empty your trash, simply specify the value "trash" for the File ID (without quotes).  If your trash is quite full, the command will return quickly but the actual clearing of the trash will take a little while from the testing and customer experiences we have.

More documentation for G4G, as well as this new function, can be found in the G4G Documentation.