I had this problem as well once. I ended up using the terminal technique but option might work as well so try that first. These will erase everything in the trash
Force the Trash to empty using the Option key
This technique uses a hidden feature of Mac OS X to force the Trash to empty. Perform the following steps in the order specified:
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.
Select Empty Trash from the context menu for Trash.
Release the Option key.
Force the Trash to empty using Terminal
This technique uses a Terminal command to force the Trash to empty. Perform the following steps in the order specified:
1. Launch Terminal, located in the Macintosh HD > Applications > Utilities folder.
2. At the Terminal prompt, type the following command exactly as written:
sudo rm -rf ~/.Trash/*
Note that:
There is a single space after each of the terms sudo, rm, and -rf in the command.
Assure you have typed the command exactly as written before proceeding: typographical errors in this command can have dire consequences, including erasing your hard drive!
3. Press Return.
4. Type your Admin password when prompted, then press Return. The files in the Trash are now deleted.