If you are using the free Spark Starter plan, the Adobe Spark watermark and the "Made with Adobe Spark" slide will appear on all Spark Videos. With the full version of Spark, you will have the ability to remove the watermark and customize the final slide.
If you are using the full version of Spark and you would like to customize this Outro slide, first set up your brand by clicking "Personalize this design" at the top left of the "Your Projects" page. If your brand is already set up, click "Manage Brand". From there, scroll to the bottom and click either "Video Stamp" or "Video Outro".
If you have the full version of Spark and you would like to remove the stamp/watermark entirely, simply click the stamp/watermark while editing a specific project, and toggle "Show Stamp" to off.