as title

If you decommission a peer node (as in, unplug it), there's not much else that is required. If you want to keep the peer running but you want it to "abandon" a channel (as in, stop processing its blocks and delete associated ledger and state DB), unless I'm much mistaken this feature is not implemented. If your use case requires it I encourage you to open a JIRA for it.

Thanks,
Ale