Not tested on 6.49.8 but probably applies to this version as well:
Security Issue: Changing rights / disable / delete the users has no effect on already logged in users. [Tested on 6.48.7 and 7.10.2]
http://forum.mikrotik.com/t/security-issue-changing-rights-disable-delete-the-users-has-no-effect-on-already-logged-in-users/168574/1