The solution is to disable the speed warnings altogether through the menu settings. You can disable the sound and leave it visual only, or disable totally, IIRC. That's one of the reasons I dislike the Mazda sat nav and went over to using Waze, that's always up to date in this respect.
Whilst it's probably better to have alerts for cameras that are no longer there, it also means that you may not have alerts for cameras just installed as well...
Alan