The reboot works only on failed monitors. You would need to change the logic of your filewatch to be successfull (green) if a specific content is not machted. However i havent seen a way to negate a regex in ipmonitor yet (normaly you would lead it with an exclamation mark !). Even the regexwizard does not have a "not" option.
Check out Re: SmartGroup using negated RegEx for how to negate regexes in IPM. Also there's a great utility mentioned (Expresso) for building regexes. Thanks to Peter Cooper for both of these.