How to REMOVE a RELATIONSHIP (values in a LOOKUP field) using a STATELESS FORM

How to REMOVE a RELATIONSHIP (values in a LOOKUP field) using a STATELESS FORM

I have a stateless form to add records to "Tasks" form and a separate stateless form to edit records in "Tasks" form

I do not want to allow users to have edit access to the actual "Tasks" table, so all edits must be done via the stateless form.

"Tasks" form has lookup field Contact_ID which links to "Contact" Form.

IF a contact has been added into a record in "Tasks", i.e Tasks.Contact_ID has a link to Contacts.Contact_ID, how can I remove this relationship on edit, using a stateless form?

None of the following work because of the pre-existing relationship between Tasks.Contact_ID and Contact.Contact_ID:
Tasks.Contact_ID = null
Tasks.Contact_ID = 0
Tasks.Contact_ID = ""
Tasks.Contact_ID = "-Select-"
Clear Tasks.Contact_ID

Can anyone advise the correct deluge expression to CLEAR a value in a field as opposed to setting it to null or 0?