How to clear/null a reference combo box

I've created a combo box/reference that will allow me to create tasks for a record from a list of selections. The field in the main record "Project" has a combo box called "Tasks to Apply" and when a user selects a record from the list, a number of tasks are created in another table. What I am wanting to do is clear/null the Combo box out since the tasks have been created, however there is no "field" that exists for the combo box, it is a table reference and any clue how to null out the combo box selection?

My task creation was baed on: http://software.kirknessassociates.com/blog_files/e7d4179b07c6bcb203ae50a5b26e828f-42.html