Just the normal "status" field we have for other reasons like "ready, planning, in progress, testing, etc". Another one says "hold". Along with that we either delete or push out the planned work so as not to affect resource capacity.
We also keep on hold projects visible. Having it present means someone has to do something... get it started again, formally cancel it, what have you. Cancelling or completing is what makes the projects come off of most of our views.
