When a Content Editor is given access to edit a custom form, they also have access to edit, add, and remove the fields on that form.

Removing access to those tasks by the Content Editor will only allow them to manage submissions for the form and not edit it.
  1. Core > Security > Roles
  2. Click on the Content Editor role. 
  3. On the left, click on Tasks. 
  4. Click Edit in the top-right.
  5. Uncheck all boxes for these tasks:
    • Manage Forms Category
      • Add Form Item
      • Edit Forms
  6. Save & Exit.
Note: When you add or remove tasks for a role, the members of that role must sign out and sign in again before they see the results of those changes.