Phone is roaming to US but I'm still in Canada

Go to your network settings and change your network from Automatic to Manual and then manually select Koodo.  This will prevent your phone from connecting to non-Koodo networks.  Note you may need to reapply this setting after a phone restart

White Rock is close enough that your phone can latch onto an American tower without you going anywhere, that's what triggers the roaming reminders. The notification is a generic notice though, it doesn't mean you're actually being charged.

To make it stop, turn off automatic network selection in your phone settings and manually select Koodo so the phone stays on the Koodo network near the border. I would turn data roaming off as well. Koodo covers this in https://www.koodomobile.com/en/help/understanding-how-youll-be-charged-while-travelling

Also, which phone are you using? the menus are different on iPhone vs Android.

Have you checked your usage in Self Serve? If an Easy Roam charge does land on an issued bill, you can submit it through Ask us to review a charge, accidental roaming like this normally gets credited https://www.koodomobile.com/en/help/ask-us-to-review-a-charge-on-your-bill