Under what conditions does this happen, e.g. everytime you log in? Or while you click through menus? Or when something is done on the server such as the nightly maintenance? Does your server have more than one administrator?
You'll need to go through the existing entries and validate, if there is one custom job that runs "plesk bin poweruser ...", e.g.
in GUI: Tools & Settings > Tools & Resources > Scheduled Tasks (Cron Jobs)
in CLI: /etc/cron* directories and /var/spool/cron/* files
It's only a guess.