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.
Anyone reading this thread (users who have older versions of AEM for example) and want to know HOW TO use editable templates within AEM - see Adobe Experience Manager Help | Creating an Adobe Experience Manager 6.4 website using Editable Temp...
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 ?