To remove a Form from your dashboard the workflow owner just needs to set "Publish Google Form on the Web App" to No on the Setup menu.
If a Google Form has been permanently deleted from Drive but is still showing on the Web App you simply need to remove the spreadsheet from the dashboard. Once the spreadsheet is removed the permanently deleted Form will also be removed.
My Form is no longer linked to a spreadsheet
If a Google Form on the Web App is no longer linked to a spreadsheet please proceed as follows:
1. Search the spreadsheet name in your Drive, as it appears on the Web App
2. Get the Google Form Edit URL by searching for the form on your Drive then click on the search result.
3. This will open a new tab where you can get the URL, as shown:
4. Run Form Workflow Plus on the spreadsheet > Link an existing form
5. Paste the Google Form URL and click Link Form, as shown:
6. On the Advanced Settings tab, set Publish Google Form on the Web App to No, as shown:
7. Refresh the Web App: https://app.formworkflowplus.com/requestforms.
Requesters will no longer be able to access the Form from the Web App