This kind of toast notification is very annoying, especially when you are using a navigator. It is not listed in the notification list, available in Windows 10 Mobile, and so it seems not possible to disable it!! On the contrary, fortunately you can: you must go to Settings -> Network and wireless -> Wi-Fi, click on the Wi-Fi sense button and then turn everything off.

Wi-Fi sense

Personally I think that currently you have to disable things that are not necessary related to the fact that you do not want to receive a notification when a new wifi is available … it should be much better if this could be managed in the Settings’ notification management section, possibly choosing the kind of notification you might like (e.g. sound, banner …).