You can remove the Emails related list from the appropriate page layouts. This may not be ideal because in rare instances comments are not converted into Case Comments properly. Additionally, users might want to view the HTML version of an email and they need access to this related list in order to do so.

 

The New Comment page uses Apex to send emails and Apex runs in a context that supersedes users' permissions. Therefore, the "Send Email" permission can be disabled for any given profile and the New Comment page will still function properly. The "Send an Email" button still shows on the Emails related list, but it displays an insufficient privileges error if clicked. The caveat to this approach is that users will not be able to send emails from other page layouts in Salesforce.

Option 3: Disable "Send An Email" button on Activity History related list
If you choose option 1 above and still want to include the Activity History related list, you can hide the "Send an Email" button.  The "Send an Email" button will no longer be visible on the Activity History related list once it is unchecked from the related list properties wrench in the page layout.  This will allow the user to see all activities for that case, but no longer be able to send an email using the Salesforce built-in email page.