|Ok, again, you have to go into the routers setup. Mostly you have to open your web browser and type in 192.168.1.1|
Then you have to type in a username and a password, that you have to figure out in the routers manual. Username is normally admin, but default password differs (READ THE MANUAL).
The go to the wireless configuration page and switch off the SSID broadcasting.
But again, everything can be read in the routers manual!