Clocking in does not mean you have accepted a patient assignment. Clocking in is reporting to work.
Most if not all BONs leave it up to the NURSE to determine if an assignment is safe before accepting, because should the nurse choose to accept an assignment that he or she feels is unsafe, the nurse is liable for anything that may happen. You need to find out where your BON stands on this.
In many states, if you do not accept an assignment, there has been no nurse-patient relationship established and thus there is no abandonment. This doesn't mean that an employer can't fire you for not accepting that assignment: they very well can do so, and do so legally in most cases. But that would be an employment issue, which BONs as a rule stay out of.
IMO, it is far easier to find a job if you've been fired in the past, than if you have BON discipline on your license.
Call/write/visit your BON and see what they have to say, as it's what they think that matters the most. In the meantime, get your malpractice insurance renewed ASAP. And it wouldn't hurt to consider getting legal advice depending on how things play out. You can find lawyers experienced in nursing matters, including dealing with BONs, here: http://www.taana.org