If you are an owner or manager in a collaborative inbox, you can delete messages in the group. Note that there is no Bin in Google Groups - once you remove a message it will be deleted permanently and cannot be recovered.
To delete an entire conversation in a group:
In the group inbox, position the mouse over the icon of the sender of the message you wish to remove
Click on the check box to select the message
Click on the Delete icon in the command bar along the top of the screen
You will be asked to confirm that you wish to remove the conversation - click on OK to confirm
To delete a single message in a conversation:
Click on the conversation to open it
Click on the message you wish to remove
Click on the ellipses on the right-hand side of the message, then choose Delete
You will be asked to confirm that you wish to remove the message - click on OK to confirm