Hi Ray,
Thanks for reaching out to us here on our support forum.
There are a few possibilities, all of them will require you to have a bit of development knowledge. I will point you below to the one I found works best.
Our current code allows you to filter which are the WP_Query arguments that you use to grab the related posts, you will need to hook to tribe_related_posts_args.
You will need to take a look at the Codex page about WP_Query to be able to do what you want, check the section about tax_query