Updating this now since things have changed. Accounts can be deleted and any posts you made will become "anonymous". However, consider if you really do want to delete your account - leaving it here does no harm, you will not be contacted any more.
By far the quickest thing to do is simply stop visiting if you don't want to use the site any more. You can blank out your profile by clicking your username in the top right then the "Edit my Profile" button. Also make sure to uncheck the box saying "Allow users to post on your wall" so you don't get any emails.
If you really want to delete your account, then please email me (address is on the About page) and I will manually delete it. You must email from the address in your account (otherwise anyone could email me claiming to be you).