Races and categories

Having been in a couple of categorised races with riders who have produced 2.8wkg upwards in cat D races o think they should be immediately taken out of that race and let the legit riders have a go

Yep, sign up for Zwift Power and those Category cheats and will get removed from the results.
They are not; however, removed from the race or the next race so their presence does affect the race dynamics.
You have to know your limits and ride your race and not let them blow you up.