Accounts cannot be deleted - see this answer http://www.geograph.ie/faq3.php#103
If you have submitted no photos the only way anyone can find your account is by trawling through every account number - in November 2015 there are around 122100 of them.
If you are still concerned about your name or email address being available you can edit your profile to anonymise your name and introduce an error into your email address (which is not visible unless you have set it to be so).