This looks like google font imports via the theme options, are you sure that this font is not selected anywhere inside the theme’s options, a page template, a plugin or the themes’ functions.php file? It MUST be in one of those locations. If you can’t find it, use this (you gotta find out the handle for this, and there are different ways to do so, for example like this)
EDIT: Dequeue_style probably won’t work, as this is not a proper stylesheet being enqueued (which you can verify by checking the handles of enqueued stylesheets as described above). Focus on finding out where it comes from, it must be one of the options above; and simply remove it from there. You may also check the mentioned possible sources for the parent theme. So, what I would do is:
Check both the functions.php file of the parent and the child theme for google font imports.
Check page templates of both for google font imports (check if the font is present on all or just on specific pages, this could help you in limiting your search).
Inactivate plugins one by one and check if the style is still present or not.