Howto
Purge data from uninstalled app Univention-Dashboard
Step 1
Uninstall the app through the appcenter or use command line to uninstall. Make sure the app including the Dashboard client is uninstalled on every server.
Step 2
Purge the data directories by removing the directories:
rm -rf /var/lib/univention-appcenter/apps/prometheus/
rm -rf /var/lib/univention-appcenter/apps/admin-dashboard/
Now the app and all data is purged and you can re-install the app if needed.