Eliminate users from adding tags to posts

I run a site that has a large number of users who contribute their own posts, and lately some of them have been adding h1 tags to their posts. I want to avoid having multiple h1s per page (the article title is already set to display as h1). I’m wondering if there’s any way that we can either have any instance of an h1 tag automatically eliminated from the post content, or if we can have any instance of h1 automatically replace with h2? Or something along those lines… We want our users to be able to customize the HTML of their posts, but we also want to keep our site tidy in terms of header tags and want to put something in place so that instances don’t slip though the cracks.

Thank you!