You can also set UnreserveDays in prime.ini. See undoc.txt (search for unreserve in that file).
(21203177 is still not reserved since the server has been handing out smaller exponents, but those will be gone in an hour, so I agree that it would be best to leave it. Remember that the status files are updated only at the full hours. However, I take no responsibility for that advice in case 21203177 should turn out to be the next Mersenne prime.
)