But the argument is about due process: are we actually sure that an Bob is a a Nazi, or did him neighbour create a fake twitter account to impersonate him? Is it his namesake that he never met?

What if Steve is Bob's conpetitor and conveniently accused Bob of being a Nazi?

Suppose Bob is a nazi but hasn't commited any crimes, how far can we go, what else can we deny, can we refuse them a website? A mobile phone connection? electricity and running water? School for his child?