you want:

SELECT * from `actionlog` WHERE (`username` LIKE 'ry%' OR `address` LIKE 'ry%' OR `action` LIKE '%ry%' OR `datetime` LIKE '%ry%') AND `username` != 'cron@localhost' ORDER BY `datetime` DESC LIMIT 0,25

yours is currently like:

SELECT * from `actionlog` WHERE `username` LIKE 'ry%' OR `address` LIKE 'ry%' OR `action` LIKE '%ry%' OR (`datetime` LIKE '%ry%' AND `username` != 'cron@localhost') ORDER BY `datetime` DESC LIMIT 0,25