What I would do:
First attempt:
1. Go to system preferences and TURN OFF Time Machine
2. Reboot
3. Now try to empty the trash
If that doesn't work, I'd try this:
1. Boot from an external drive (bootable to the finder)
2. Once booted, click icon for internal drive, go to "get info", and put a checkbox into "ignore ownership on this volume".
3. Next, I'd use something like "Pathfinder" to make normally-invisible files VISIBLE on the internal drive
4. Then, I'd poke around for any folders named "trash" or "trashes", open them, and delete everything inside. I wouldn't delete the folder itself.
5. Then I'd power down, disconnect the external drive, reboot from the internal drive, and see if that worked...