You can tweak how often collectinfo.pl runs by going into System Settings -> Virtualmin Config -> Status Collection -> “Interval between status collection job runs”. That just updates system statistics, you can certainly make it once every 12 hours.
The monitor.pl is due to the “Status Monitoring” feature. You can disable that feature in your various domains (in Edit Virtual Server -> Enabled Features). Once it’s disabled, you can disable the Virtualmin status monitoring feature in System Settings -> Featurse and Plugins.