To hide a comment or message, click the client and/or partner icons to turn them black. This will prevent the clients or subs from seeing the item when they log in to CoConstruct.
If the client or partner is set to receive alerts, they would have gotten an initial email about it, but they won't be able to see it anymore within the site. Only builder side users will still be able to view the comment or message.
CoConstruct does not have a delete feature, to protect both you and your clients on the project and to preserve the paper trail.