How do i remove 'zero kb' files/folders from trash?

First, see if the trash will empty this way: Press and hold the mouse button on the Trash icon in the Dock. The context menu for Trash will display. Press and hold the Option key or the Shift-Option keyboard combination, Select Empty Trash from the context menu for Trash. Release the keys pressed and held in step 2.

To Clear stubborn Trash. Download Trash It! for Mac

Trashit is an excellent free app for the job you are trying to do.

And here is some more reading. Solving Trash Problems

If you still have the original you were copying over on the internal of the Mac--you don't want to do this unless you do--and what your were copying over resides now in the Mac's Trash, then copy/paste only the following into Terminal in Utilities, which will nuke the Mac's Trash. A typo can be disastrous, so only copy/paste. sudo rm -rf ~/.Trash Hit return and you will be prompted for your admin password, which won't be displayed in any form. You will also get a scary warning about using the sudo prefix. Hit return again and the entire Trash of the Mac, including the Trash folder, will be nuked. It will take some time to empty the 40GB, so wait until you get a new $ sign prompt, which will indicate it's finished. When you do just close the window. To restore the Mac's trash folder, not the contents, just log out and back in.