Thank you for your post.
FileMaker Go does not have a setting to lock the rotation of the layout. The Zoom Level only controls the zoom settings.
I recommend you enter this suggestion into our Feature Requests web form at:
These web entries are read by our Development and Product Management departments where they are then discussed and considered for future releases. I could copy your post and paste it into the web form, but there are a couple of questions asked that only you can answer.