To delete an empty block from your post layout, just click on the block and hit the 'delete' key or right click and select 'remove'.

If you always see empty blocks at the end of your post, this is from the automatic layout guide which helps guide you through making your post by suggesting placeholders for your next image.

If you want to stop seeing these empty blocks, you can turn off the automatic layout guide in your preferences. Head to Narrative Publish > Preferences, and untick the Automatic Layout Guide option.