Have you seen "Web Access Rules Buttons" at https://www.odoo.com/apps/modules/9.0/web_access_rule_buttons/

This addon disables the Edit button on the form views if the user cannot edit the current record according to the record access rules.

You can add this in your form views:

<field name="context">{'form_no_edit':[('state','=','done')]}</field> 

You can also put in your definition view:

<form string="Some text" version="7.0" create="false" delete="false">
</form>

With this you can hide the create/edit button and the delete option in the More dropdown button.

This option will disable the buttons for everyone.  Maybe Ray's solution it's more suitable for your needs.

:)