In AEM 6.3 - you should use Editable Templates and then define the components that are allowed in the Layout by using a policy. Once successfully done - the allowed components will show up in the editable template layout section.
Authors should not be allowed to modify template policies.
If author go about & consequently modified the policies to add couple of additional allowed components , is there any way to restrict (except for one component) addition of others by displaying a popup mentioning that " This component could not be added " ??
or is my approach wrong ?
Also it would have been helpful to know , where the policies are stored ?