Hi,
I was able to delete all the "useless" applications in the plesk database using phpMyAdmin.
Just in case someone needs to do do this as well (after he was clicking on Search" in the list of applications) here is a quick description:
Go to Tools and Settings -> Database servers
Click on the tool icon after the MySQL server in the list to open phpMyAdmin
Select the Plesk Database "psa"
Check the entries in table "ComposerInstances" and delete the items who came in accidently (in my case all items as my Joomla Installations are not listed there)
This has cleaned up my "Applications" window a lot and I can only see my installed Joomlas in the list (as it should be).
Best regards
Onki