Can we block the wifi so that others don't use it?

Your WiFi network is protected by a password.
The only way to ensure that someone who previously had access to your wireless network no longer has it would be to change the WiFi password.
You can follow these steps to change your WiFi password:
1) Login to your VoxSun router. Your router's IP and administrator username and password is provided on a paper that was included in your router's box.
2) From the menu on the left side, click on "Wireless".
3) Click on the "Security Profiles" tab at the top of the screen.
4) Click on the default security profile or the one that corresponds to your network.
5) From within the form, modify the "WPA2 Pre-Shared Key" to the new password that you want to use. (If you have WPA version 1 security enabled, you'll also need to modify the "WPA Pre-Shared Key" field.)
6) Click on the "OK" button to apply the change. If you were previously connected by WiFi to the network, you will be disconnected and will need to reconnect to the network with the new password.