You can disable the PHP mapi module by removing the
/var/www/conf/php5/mapi.ini symbolic link by
issuing the following command: 
    # rm -f /var/www/conf/php5/mapi.ini

To completely deinstall zarafa-webaccess you need to perform the
following steps as root:
    # rm -f /var/www/conf/modules/zarafa.conf
