There are a couple of ways to get rid of the confirmation message.
Option 1: Access the Clarizen settings and select the system settings tab, then click on InterAct Settings (group #5). There are three settings there, one for the lifetime of the confirmation setting (not relevant for you) and then one each for enabling/disabling confirmation messages for Email to Case mailboxes and for all other mailboxes. You can just set these to false (and then make sure to click "Save" on the setting page) and this will eliminate all confirmation messages.
Option 2: When creating "General" InterAct mailboxes you will see a setting for the confirmation message setting per mailbox that will allow you to keep as set in the system settings, disable, or enable. This allows you to turn confirmation messages on/off per individual mailboxes.
Hope this helps :)