Overriding post’s font-family

You could run a regex. Since they’re continuing this behavior, you will probably want to create a custom plugin and set up a cron job to trigger it periodically so it will continue stripping all inline styles.

Your plugin will need to query all posts. From there, in a loop that checks each post’s content, use

preg_replace('style="*.*?"', '', $content);

which will replace each inline style with nothing (”), essentially deleting it.