Hello Shirinmtmd,
Welcome to the community!
You should note that merely receiving that notification does not necessarily mean you've incurred roaming charges. I was able to confirm that situation to be true here. It is provided to inform you that your device had connected to a foreign network. I understand you did not ask to enable Fido Roam, however, it is already enabled on post-paid plans. There is nothing to turn-on, per se. Enabling roaming data only allows your phone to use data when roaming. Disabling roaming data does not prevent roaming for voice and/or messaging.
In order to prevent roaming, it is generally recommended to enable Airplane or Flight mode on your device. You can then manually enable Wifi when required. You can refer to that link above for some additional information regarding using your phone abroad.
You should also note that this forum is community-driven and not intended as a venue for customer services. If you wish to opt-out of Fido Roam, you will need to contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀
Cheers
