I presume this is the relevant image/its edit page:
You must go to the image's edit page to remove the free-space page.
Scroll down the page to the "Edit Image Details" section. In the "People & Things in the Image" subsection you can select the radio button to "REMOVE from image" the free-space page.
If you want to keep the image in WikiTree's database, you will need to attach the image to a profile or another free-space page. Unattached images are eventually deleted from the database.
Post back if you have any trouble removing the free-space page.