hoffmcs is right about channel updates and forcing and update with the channel store.
A few details about delete.... If the user deletes the channel on the box in the channel store, it will be removed from the box. If the channel was a one-time purchase channel, the user can re-add the channel with a $0.00 price.
If the developer deletes the channel from their developer "manage channels" page, it does not delete the channel from all users boxes. The channel will no longer be able to be added though... You may choose to "fade out" your channel by uploading a new version of the channel that just displays a "channel discontinued" message and let most boxes update to that version before actually deleting your channel.
--Kevin