Followup of #1849: Ability to disable channel. remove-channel will not allow you to remove it in case there was at least one build done there. It makes a sense for audit reasons, but some channels could be just disabled (test archs, etc) as they will never be used again and are just cluttering the interface and confusing users.

If channel has no builders attached to it, it could be disabled, so it is not visible in web-ui (and list-channels --disabled can be used to see them)
koji [disable|enable]-channel