You can “undo” a request for an onboardee to recomplete a form by making their previous submission the current one.
First you will go to the forms tab on their profile and select view all submissions.
This will open a window where you can select the previously completed submission.
This will allow you to set the old submission as the current one again.
If you do this within fifteen minutes of reassigning the form, the onboardee will NOT receive any notification that you reassigned the form in the first place.