Zyxel NSA-2401 Guide De Démarrage Rapide page 20

PROBLEM
I cannot turn off the
NSA.
Refer to the User's Guide for further troubleshooting instructions.
20
ENGLISH
CORRECTIVE ACTION
Press the power button in and hold it until you hear a beep. Then
release the button. The SYS light blinks red until the NSA is shut
down.
If the above shut-down procedure does not work (which may
happen if the NSA has crashed), hold the power button in a few
more seconds until you hear a second beep. The NSA turns off
immediately.