This works better

public void deleteUser(String username) throws UserNotFoundException, SharedGroupException
{
User user = getUser(username);
userManager.deleteUser(user);

rosterManager.deleteRoster(server.createJID(username, null));
}

This is the change I am making. To iterate the roster of the user and delete all roster items before deleting the actual user and thereby ensuring DB referential integrity.

public void deleteUser(String username) throws UserNotFoundException, SharedGroupException
{
Roster r = rosterManager.getRoster(username);

Collection<RosterItem> rosterItems = r.getRosterItems();

for (RosterItem rosterItem: rosterItems)
{
r.deleteRosterItem(rosterItem.getJid(), true);
}

User user = getUser(username);
userManager.deleteUser(user);
}