Hi Filippo,
You didn’t find it in the docs because it is not the right way to do it. I suppose the UI has a bug that prevents you from removing the offline node.
Having say that, to remove a node from the command line run this command (assuming the offline node is 2):
api-cli run remove-node --data '{"node_id":2}'
Added a card, to see if the issue is reproducible. See Trello.