1. From Setup, enter Chatter Settings in the QuickFind box, then select Chatter Settings.
2. Click Edit.
3. In the Post and Comment Modification section, deselect Allow users to edit postsand comments.
4. Click Save.
5. If you have assigned the “Can Approve Feed Post and Comment” permission, disable that on the relevant permission sets.“Can Approve Feed Post and Comment” lets assigned users edit and delete any post or comment in any Chatter feed those users have access to. This permission is available under Setup | Permission Sets | Select a permission set | System Permissions.