If you have shared a link to your mind map, you can stop sharing that mind map by "deleting" that link.
To do this,
- Open the share dialog
- Click on the "Link" option
- Click the "Delete" button next to the link you've shared.
Now the people with a link that was deleted will not have access to your mind map.
To share the mind map again, click on the "Create" button. This will generate a new link to your mind map.