Welcome to Canada, dear OP.
As someone who has been on both sides of this conversation, I can understand. As you already surmised, these queries do not stem from malice but rather genuine curiosity. And as made blatantly evident, the repetitious nature of it, which the inquirers are unaware of, is grating.
Most people simply want to connect and want to impress and be appreciated and that is why, cheesy as it is, they try to speak to you in your native tongue. When people have tried it with me, it annoyed me but I understood. Then again, unlike you, I did not have to put up with it ad nauseum which would definitely change my temperament over time.
As a Canadian, I will tell you this- Telling people that you are from Canada is a subtle but stern enough message for most Canadians. However, the other suggestion of pinning a spanish flag badge would also be an alternative. Mind you, however, you might still get asked "what country" does that flag represent for the customers not in the know.
Good luck.