Showroom is Shared with You
If you are the user invited to the showroom, you can simply remove yourself. See Remove a user from a Showroom.
Showroom is Shared with Teammate
If you see a "Team" icon instead, you can click it and see the teammate who is invited into the showroom. You have a couple of options for removing this showroom from your list. See Different Showroom appearing in Showrooms Panel.