As site admin, I would like to globally default the ability for people to post comments under a classmate profile to default NOT to allow. I don't mind letting people change it individually to allow. I don't see where to do this, after a bit of poking around. Advice? Can it be done?

Welcome Elizabeth.

As you pointed out individual Classmates can allow or disallow that on their own Profiles, but it's not a global admin option. I suppose we could make it an option though. I don't think anybody's ever asked. Can I ask why you want to remove this ability? This is one of the things that many Classmates enjoy the most. Of course, if an Admin wants to read absolutely everything posted on their site, this can also be a lot to keep up with. Is this why you're wanting to remove that feature?