Get into the device via a web browser using the default gateway IP address provided by your ISP. It may be 192.168.1.254. Once in, on the left you will see a row of buttons. Choose the "Wireless" button. You will probably be prompted to enter the Device ID number which you should find on the bottom of your 7550 device. Enter this number and you will be taken to another screen.
On this "Wireless Setup" page you will be given the option to DISABLE the wireless operation. This should do it. Of course this will shut off any wireless access to this device.
However, if you are trying to just disable the wireless security but keep the wireless connection, this cannot be done on this device. You can hide the SSID, (this option is on the same Wireless Setup page,) but the device will require a Network Security Key.