Thanks for your hints!
Revo Uninstaller Pro gives a comprehensive overview of most W7 settings. In the set of Auto-run programs, I saw a few with incomplete reference information, e.g. SmartView, which accidently has been installed on my computer. I know that, I’ve declined installation of that program! When said program was uninstalled, all four ZYNGA game-bars disappeared from all accounts, so the problem is solved!
PS. ZYNGA support hasn’t provided any useful advices! DS