By users did you mean Staff in your store?
If so, you can set permissions for each Staff account in your store by going to Settings -> Users and permissions
You should make sure that the staff account does not have the Themes permission enabled so that they will not be able to edit the store theme
Let me know if this answers your question!
Ok got it, so you are essentially looking for a way to lock your current theme layout.
Currently there is no way to lock your theme layout but you can configure presets and defaults for sections and you can also restrict a section to a specific template (but blocks and sections will still be editable in the theme editor) you can find the docs here: https://shopify.dev/themes/architecture/sections/section-schema#app-block-schema
Let me know if this helps!