It depends on how your sessions are stored. If they are in the database, just delete them. If they are on the file system somewhere, like in / tmp, just delete them. For session information in PHP, see the manual. http://www.php.net/manual/en/book.session.php
session_save_path(), , , memcached, mysql . PHP . ,
Shell: rm -rf /var/lib/php/session
PHP: shell_exec('rm -rf '.session_save_path() );