Add everyone onto a static list. Remove them from the program by saying status is "Not in Program." Then readd them to the proper status.
Be aware of double scoring your webinar registrants if you have status-based scoring campaigns in place. They could be scored for attending the webinar and for their new status depending on what your scoring campaigns look like. This can be rectified with a "Change Score" flow step.
I moved your post to the Products and Support
Favor to ask: Can you mark which answer you think is Correct Answer! (You will get points and so will the person with the correct answer : )