As a paid member, you can ‘downgrade’ your membership to a free one. See:
http://ancestry.custhelp.com/cgi-bin/ancestry.cfg/php/enduser/sab_answer.php?p_faqid=891&p_created=1013541247&p_sid=14XPYJyk&p_accessibility=0
However, being a free member, your account cannot be deleted, as explicitly stated on:
http://ancestry.custhelp.com/cgi-bin/ancestry.cfg/php/enduser/sab_answer.php?p_faqid=577#res2
URLS:
http://www.ancestry.com