Hide ability to 'Comment' from unprivileged/SelfService users

Hello,

Currently all of our RT users are unprivileged members of the group
’employees’ that have the following global group rights:

User Defined Groups

Employees:

CreateTicket

DeleteTicket

ModifyCustomField

ModifyTicket

OwnTicket

ReplyToTicket

SeeCustomField

Although the ‘employees’ group does not have the global right
"CommentOnTicket" the users in this group (via SelfService) still have
the ability to comment on tickets. What is the proper way to disable the
ability to ‘comment’ on a ticket? We’d like the users in this group to
only have the ability to ‘reply’ to tickets.

Thanks.

-Matt