You can short to lock in your margin account sharesShares that you own will be held up until the institution receives them. This is likely the case for all retail brokers. As far as I know it is true for TD and Ameritrade, no doubt the rest. It takes two to three days from what I'm told.
Institutions themselves will likely have first crack at selling what they own.
If it spikes and you want to lock in the price, use your short account. TD will allow you to short the equivalent number of shares that you have in your margin account. Other retail brokers will likely allow it.