Solved Moved all topics from Category - but howto remove category?

I have moved a topic and all answers to another topic.
But the old topic is still visible, but clicking on it will show me the new topic.

How can I completely hide the old topic from the overview?

I have moved a topic and all answers to another topic.

Have you merged this topic with another topic, or moved the topic to another category? That's not the same.

I merged the topic into another topic .. sorry for confusion

If you have forgotten to mark the checkbox "Set the subject for all replies" remain the old topic title. Open this topic and click on Moderate Topic. Then mark this checkbox and enter the new subject.

I cannot open the topic anymore .. if I click on this topic then I will automatically redirected into the new topic.
I changed the title in all entries in new topic, but the old topic title still visible in overview.

I think, you've created a shadow topic. If you move a topic, you have a checkbox for "Leave shadow topic pointing to new location". If this option is chosen, it remains a shadow topic on the old location with the old title and if you click on it, you get a redirect to the new topic.
I've created an example for you.
Moved the old topic to the new topic (with shadow topic). Now I have the same as you (see image), an old and a new topic. The old topic is only a redirect link and can be deleted.
But beware! Delete this entry not immediately permanently, test it before.