You can remove a user or group from a message after the message has already been sent. However, you cannot remove a user from the message if the user has already visited the message (by clicking the Section 2.3, Replying to a Message).button next to a message), marked the message Read, or replied to the message (as described in
To remove users or groups from a message:
Locate the message for which you want to remove recipients.
Click the (x) icon next to the user or group name that you want to remove.
If the user has visited the message or replied to the message, the (x) icon is not available and you cannot remove the user from the message.