Welcome to the forum. The app has a setting to turn this off; if that doesn't do it, uninstalling and reinstalling the app might be worth a try. Otherwise, you could just turn the phone wifi off until you actually need it.
If you've upgraded to Marshmallow you cannot prevent the sound (& possibly vibration) occurring when your phone hits a wifi hotspot. It's not the BT app that's doing it. And no, until Google provide a fix or toggle, you cannot turn it off. Annoying, innit?
I also have the same issue since upgrading to Marshmallow on my Moto G 4G (I wish I never had!). Thank you for informing us it is a Google problem. Do you know if it affects other phones / wifi providers, too? I am hoping that if it does, Google will do something about it soomer rather than later.
Am I missing something here? If the bleeping is annoying, presumably you're not looking for a wifi connection. If so, why not just turn the phone's wifi off until it's needed?
A bit more research and I found that this issue affects pretty much every phone with Marshmallow:
I think I am going to get an iphone now....
Also having the same problem with my Moto G 2nd gen since updating to marshmellow.
I have uninstalled/ reinstalled the app, turned the sound toggle on and off but it still notifies me with a reconnection.
When i walk a few hundred yards from home the bl××dy thing is 'going off' 5 or 6 times.
It really is annoying.
Nobody's yet said why they don't just turn wifi off until needed.