[imp] How can destroy session

[Kakooty]

How can i destroy the session.
in my module the when the user login, check he/she change your password. 
if yes, it redirect to index.php and if not, the user must change your 
password. if the user  don't change password, the session must be 
destroy and so on...!

where can i found a refrence or faq for developer(some like my question)
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thanks mike ;)