You can lock a conversation in a collaborative inbox to stop the message from being replied to, or from further action being taken on the message.
To lock a single message in the collaborative inbox, open the message, then click on the Lock icon on the command bar along the top of the message.
To lock multiple messages in the collaborative inbox:
Position the mouse over the icon on the left of each message you wish to select, then click on the checkbox
After selecting all messages, click on the Lock button in the command bar above all messages
Locked messages show a padlock icon in the collaborative inbox.