I've done this before using a Terminal command, can't remember it myself but found this:
How to force the Trash to empty using Terminal

This technique uses a Terminal command to fore the Trash to empty. It is important that you follow these steps very carefully.

1. Launch Terminal, located in the Macintosh HD > Applications > Utilities folder or search Spotlight for Terminal

2. At the Terminal prompt, type the following command exactly as written:

sudo rm -rf ~/.Trash/*

Important Notes:

* There is a single space after each of the terms sudo, rm, and -rf in the command.
* Double check you have typed the command exactly as it written above: incorrectly typing the command can result in serious system issues, including erasing your hard drive.

3. Press Return/Enter
4. Now type your Admin password when prompted, then press Return/Enter. The files in the Trash are now deleted.