You should be able to do it on your Self-Serve account. https://www.koodomobile.com/en/help/easy-roam

Login into self serve and check your add-ons. 

Then remove the $12 US easy roam and $15 international easy roam add-ons if you have Easy Roam enabled 

@Reni Pls remember removing easy roam from your account doesn’t automatically stop roaming, connecting to the local network in the country of your travelling.  One of the better ways to do that is to leave your phone or airplane mode and/or take out the SIM card. In each case your phone could still connect to wi-fi.

To turn off international data you can remove the free international data roaming add-on. I think this will stop International data but not US Data.

You can turn off data and data roaming on your phone. But honestly data leakage can still happen. Safest way is to take the sim out of you phone like Bernard said above. 

Thank you guys.  I already removed the US easy roam and international easy roam add-ons. Turned off data and data roaming on my phone. Hopefully it will work. Will find out next few weeks :)

Becareful. Data leakage could still happen. Especially when restarting your phone. 

Here is a helpful article: