If you clear all your cookies, it will clear the MouseBits cookie as well as those for all other sites.
If you don't want that, you can remove the individual file.
For Internet Explorer:
With windows, browse to "C:\Documents and Settings\Yourusername\Local Settings\Temporary Internet Files" "Yourusername" is your Windows login account name.
Look for cookies that end with "@dc-torrents.com/" and "@mousebits.com/" and delete them.
For Firefox:
There is a cookie browser program built into the web browser. Use it to remove cookies specific to "dc-torrents.com" and "mousebits.com".