There are a bunch of Restrict Editing options. The most restrictive of which is "Filling in Forms." That is almost never the appropriate method for newly created forms. That is a control for legacy form fields.
The simplest is to select an area and apply a Group content control over it. The only things that can be edited inside such a control are content controls.
Another is "No changes ... with exceptions." You can set an exception where you want someone to be able to make changes.
Here are some resources that may help you.