What is a Stream?
A stream in CanIt is a "container" for the settings, rules and quarantines used by the filter for one or more related email addresses and is typically managed by the owner of at least one of the addresses. CanIt creates streams as they are needed and will expire streams when they are no longer in use/needed.
An active stream is one that has either settings or rules that are specific to it, and/or, it has messages trapped in one of its quarantines - Pending, Spam or Non-Spam. Active streams can be viewed by administrators on the Administration->See Active Streams page.
Note that if you click on the "(Change Streams) link on the upper right of the WebUI and enter any name in the Stream box and click the View This Stream button CanIt will show you the settings and rules for a stream of that name, even if one did not exist before this, although in this case it would be empty (show only the default values). This allows you to pre-populate settings or rules for a stream that subsequently will be processing email.
CanIt, in most cases, will remove unused streams automatically if they are not processing/filtering email, and there have been no new messages trapped in the quarantine in the last 21 days, and there are no rules and/or settings that have been created for the specific stream. (If any of these criteria change CanIt will "recreate" the stream as needed.)
In the case of Active Streams the administrator can explicitly delete them on the Administration->See Active Streams page using the Delete link on the right. This will remove all rules and setting in effect at this time, irrevocably, for that stream name and it will disappear from the Active Streams list.
Many clients are concerned about billing and believe the deletion is necessary to prevent overcharging. However, this is not the case, billing is based solely on the monthly provisioning snapshot (available to an administrator on the My Domains->My Provisioning page of the realm at the top left), which is done at the first of every month and this looks only at whether the stream received email in the previous month. So once you have disabled the email address so that recipient verification no longer sees it as valid, the address/stream will disappear from the snapshot in the month after it is last seen.