You can deactivate the built-in Wifi in the Raspberry settings. Your wifi dongle may still need a suitable driver. You can install it with the following commands, provided your dongle is connected and recognised by the system:
sudo wget http://downloads.fars-robotics.net/wifi-drivers/install-wifi -O /usr/bin/install-wifi
sudo chmod +x /usr/bin/install-wifi
Restart your system afterwards.