If you give a user access to the .indd file, they have total control. If the organization has strict style requirements, that should be made clear to the user at the start, and if they cannot follow the directions, well they sholdn't be working on the project.
An InCopy workflow might help, though I'm not sure if it will prevent the editing of styles.
The styles in an InCopy document are defined by the InDesign document they're linked to. An InCopy user can't edit them. They can, however, change which style is applied, which sounds like what you want. It might be worth your while to look into it.
My $0.02 is that if a user can't be trusted to not change styles, they'll never figure out the check in/check out process for InCopy.
Honestly, if this is an issue, find better talent to do the work.