The punishment from the Super League disciplinary committee follows a brawl at the end of the Devils' game with Nottingham Panthers last Thursday.
MacWilliam originally received a game misconduct and a gross misconduct penalty for fighting. But after reviewing a video of the incident, the ISL has thrown the book at the 31-year-old Canadian.
It means the Devils will now be without the forward in what is a crucial spell for the Welshmen. Their win at Manchester on Sunday - only their second in the last seven games - cut Storm's lead at the top of the League to just six points.
Breaking down his six-game ban, MacWilliam misses two matches for unsportsmanlike conduct, one for verbal abuse of an official, one for being a repeat offender, one for resisting a linesman in the discharge of his duties and one for collecting his fourth game misconduct of the season. On top of that, he has also been fined pounds 350.
Defenceman Frank Evans, who was given a game misconduct for fighting, has also faced further punishment. His offence has now been upgraded to a match penalty for fighting and reckless use of his stick. He has been banned for two games and fined pounds 250.
The Devils coach, Paul Heavey, also received a written warning, with specific regard to the control of his bench.Reuse content