You cannot totally remove your data from their system, so you’d better use the tip below and anonymise your data.
Then, send an email to NFLPrivacy@nfl.com to request cancelling of your account.
See http://www.nfl.com/help/privacy#data for more information.
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.