Yes — ambassadors can use their own code and earn commission on their own purchase. There's currently no setting to block it, and honestly, we don't think you should want one.
Two reasons it's usually fine, as long as you're profitable on every sale:
Ambassadors buying your product is a good thing. They need it to take photos, share it on social media, and talk about it with friends. The more they buy, the more they can post.
And your profit is the same either way. The money you make on an order doesn't change depending on who used the code.
It's also very hard to enforce. An ambassador can check out with a different email address in seconds, and blocking by shipping address just punishes family members and housemates who live with them — anyone can ship to a friend or their office instead.
If you really want to prevent it, add a clause to your ambassador terms and end partnerships with people who break it. We'd generally recommend against it: our view is that ambassadors should be able to use their own code.