Best answer by BobTheElectrician
In your preferences for [b]Mobile Networks, set your provider to [b]Koodo rather than [b]automatic.
Your phone will normally latch on to the strongest signal in your neighbourhood, so limiting your connecton to Koodo should kabosh any accidental roaming. If you do intent to visit the US, you will have to revert that setting or you will get no service at all there.
This is not a uniquely Koodo problem... changing providers will not help as long as your handset is allowed to zero in on the strongest signal regardless, and Koodo has been good in the past on reversing borderline roaming charges.