I tried using Trash It!, force empty, and several of the other methods here, which didn't work, unfortunately - for a malware application installed on my Mac which I was trying to delete.
What eventually worked was that I right clicked on the app (while out of the trash) and chose to "View Package Contents". Then, I was able to one at a time drag various folders and elements to the trash, and then Empty the trash. Some of the items would still give me the -8003 error code, so I would have to drag them back out of the trash, and then click Show Package Contents for that file as well, dragging its subfolders and items one at a time to the trash and Emptying again. The process took about 15 minutes, but eventually all the top level files which were not able to be emptied eventually were able to be deleted by this method, basically dismantling them from the inside out.