With Soapbox, you can create user-defined session variables to be used as default values for form fields across a number of apps. A value that has been saved to a user-defined session variable can be removed by including that variable in a URL string and leaving the value blank.
As an example, this URL defines a user-defined session variable:
This URL removes the value:
One potential use for this could be as a Redirect URL for a Forms app page. If you wished to remove a value used for a user-defined session variable on successful submission of the Form, you could set the Return URL for the Form to direct the user to a specific URL and include the blank session variable.