Hi Uwe,

You can set the event creation to require moderation so when users create events, it is not published and requires moderators to review them.

Another way for users that have access to the back-end is the following:

Create an event category, unpublish it. When creating new events from the back-end, select that category.

When you are ready to publish it, move the event to a published category. Make sure you are using the same form builder so that event data retains when switching categories.