This is a “note to self” type post, and basically because Google seems unable to find a direct answer to this simple question.
So it’s simple as this
(config)# interface ethernet NN
(config-if)# no channel-group
et voilà, the ethernet port doesn’t belong anymore to the port channel. It should work with PowerConnect 5324, PowerConnect 5424, PowerConnect 5448, PowerConnect 5548 etc.