You can easily stop this by manually selecting your network to Koodo when you are close to the border. If your phone keeps searching for a network automatically, it may connect to a U.S. cellular tower, which happened in your case. 

Not practical. We live, play, drive close to the US border all the time as we live in Windsor, ON. As we are driving down a road that follows the Detroit River I can’t  or anywhere close to the USA I can’t stop and manually change networks if it does connect to the USA.

I just do not want roaming charges. Clearly you have not figured out how to make this simple!

The solution is there. Switch to Manual Network Selection and you won’t have issues. You won’t see anything change as long as you stay in Canada. You toggle the switch once and forget. Just remember that when you do need to use US network, reactivate automatic.

Oh, wow... This is how you reply to someone who are trying to help your issue? 

MilkyWay suggested to do "manually selecting your network to Koodo". Once it's set, you don't have to do it all the time. Isn't it simple enough?

You should apologize Dude. The answer is correct and even if it wasn't, a volunteer took the time out of their day to try and help you.

I think you misunderstand. Manual network selection isn’t you have to change it back if it changes to roaming on a US network. Network selection can be automatic or manual. Automatic means your phone will pick what it thinks is the best network if roaming. Manual means your phone will not try to connect to a different network if it’s connected to the one you choose normally. So if you select Koodo manually one time, your phone will not connect to another network later on. Then if you want to roam, you’ll pick the network you want when you do cross borders.