Officiating at the End of Game

yikes

Every penalty is delineated and explained and the result it garners.
as it is in 8.90:
RULING: Half over. Intentional grounding and a 10-second runoff.

There is nothing in the rulebook saying you can't spike the ball with the clock not running.
again, 8.90, next sentence:
A QB can only spike the ball to stop a running game clock.