If you save the roster data in mnesia (mod_roster) and not in an external database (mod_roster_odbc), then mod_admin_extra:delete_rosteritem/4 might help you to delete the user's roster. Otherwise you will have to use odbc_queries functionality.
If the user's contacts are your users as well (no s2s connections allowed), then you might use delete_rosteritem with swapped arguments, or use mnesia/odbc hackery. In a federated setup you will not have access to the contact's roster, but you can probably send his server both, an presence unsubscribe and a presence unsubscribed stanza. See RFC 3921 or ask for detail.
Unfortunately, I do not know a ready-to-use solution, but might come up with one in the following weeks.