Posible to forgo a seat assignment online?

I just booked a flight that is now sold out. I purposefully didn't assign myself a seat (it's a CRJ-200, so not worried about middle or E-). But UAL.com automatically gave me one.

Is there a way on line to relinquish a seat while keeping the reservation?

I may not take the flight, and I don't want to block out a seat if families want to sit together.