It is straightforward to restrict access to templates using ACLs
For instance, group your templates per site and deny access to the folders that a role should not have access to.

E.g. try the following: deny access to CONFIG: /modules/templating/Templates/samples for role "editor"
Then log in as a user with "editor" role, and you will not see the sample templates.

Of course this might get tricky if you need read access to the templates, but as long as you have different sites with a set of allowed templates per site it should work fine.

Here is how you could set up a multi-site Magnolia with different templates per site:

• create role "editor" where you define common ACLs. Concerning templates:
  • readonly /modules/templating
  • deny /modules/templating/Templates

• Create roles for each site (site1editor) where you grant access the resources. Concerning templates (assuming you have three templates to be used for this site:
  • readonly /modules/templating/Templates/myTemplates/FirstSite1Template
  • readonly /modules/templating/Templates/myTemplates/SecondSite1Template
  • readonly /modules/templating/Templates/myTemplates/ThirdSite1Template
    (this method does NOT work with trailing slashes e.g.: "/modules/templating/" )

or group templates by site to make it even easier:

• • readonly /modules/templating/Templates/myTemplates/Site1 (where the Site1 directory contains all templates for Site1)

• create site1 users and give them roles editor and site1editor