and ISPconfig sees it as 1 not to be updated.
As I posted already, ispconfig is not updating anything. What you see in the ispconfig monitor is the output of apt of possibel updates that are available for download on the ubuntu servers, it is not something that will get updated or even installed.
So your solution goes into the wrong direction. Instead of running a cronjob that holds the package, you should find the software or cronjob that is actually installing it automatically and disable this software. One candidate for this is apt-cron, so check if apt-cron is installed and uninstall it. If you use a desktop system, check also the desktop package installer and applets.