The rules say:
3. Crew Losses: For each complete crew section wiped out or used for boarding, the Hit Table is decreased by one. If no crew sections are available for firing, the guns may not be fired.

http://www.youplay.it/beta/play/ws_GameMove.asp?GameID=33

During Turn 32, BON fired at SER with only 1 crew on 6 (2 section loss) but only a -1 penalty was applied.