When building a Form (Beta) block it's only natural to want to test the form out. Once you're ready to release the app to your users, you can delete any test demo so that it doesn't muddy the data collected from user responses.
- Click on "Developer" in the left vertical menu
- Expand the "Custom Objects"
- Click on block name
- When you hover over the record, you will see an "trash bin" icon to delete the record.