Some ISP's keep it in their system indefinantly so that they can determine how "fair" of a customer you are if you attempt to rejoin. If they just deleted accounts right away any yahoo could join their service with a back balance and avoid charges.
aol usualy i thionk keeps your account details for about 6months and if you dont re-join in those 6months they delete you off the system and iff you then after 6 months decide to re-join you have to register all over again
