I don't think you can remove them from the list results. At least you change the validation handling the results.
What you can do is for example to create a new category (like the ones used when creating a new portlet: Request, Packages, etc) and put those especific portlets in the category you created. So, you can sort them and ask customer don't use them. Or as I said above modify the validation returning the portlets and exclude the ones you have in the new category.
Also, there is an ACCESS tab on every portlet definition. There you can define who can have access to an specific portlet using the security access options.