I don't have an X-T3 to check and it doesn't mention in the manual but its possible that you can lock the dial using the LOCK option in the Button/Dial Setting menu
OH! I hadn't even noticed that the dial had moved from S! Thank you! 🙏🏼 This is something I hate about this camera - this dial moves on its own, you can't fix it so it doesn't move onto a different setting! 🤦🏻