You cannot delete your account.
From their FAQ:
As stated in our User Guidelines, we do not delete accounts on WSO, however, you may change your username to something non-descriptive and request the removal of any comments you feel are a risk to your anonymity. Please email [email protected] with these detailed requests. If you would like specific comments removed, please include a list of the comments you would like removed for consideration.
URLS:
http://wallstreetoasis.com
Please note that AccountKiller.com is not associated with the business listed on this page. AccountKiller provides easy-to-understand instructions and guides on how to cancel charges and relationships from the list of businesses on our site, we have no affiliation with these businesses.