To disable wifi by default on boot, one option is to add a command to rc.local to disable it.
Run in sys-net terminal the following:
sudo gedit /rw/config/rc.local
If sys-net is disposable, then run the command in the Disposable Template (the underlying template for sys-net). If gedit is not installed, use another editor such as nano.
Then add the following text to rc.local and save:
sudo nmcli radio wifi off
This will disable the wifi on boot. It can be enabled easily via the Network Manager (check box).