If you no longer wish to play on GreedyGammon, you may uninstall the software from your computer by going to Start->Control Panel->Add Remove programs. No formal cancellation is requred as it is a free service. If you are concerned about your email listed in the forum, you can log in and change it in your profile. This will ensure that you never receive any email notifications from GreedyGammon.
Hope that helps,