You can ban...
The reason why things are banned. People in power can't tell who will use them responsibly.
"I'm not Santa Claus. I can't figure out who's naughty and who's nice," he said. "So we went ahead and banned all of them."
Of those users that use the banned item, and are irreponsible
Banning is an archaiac Victorian attitude, that has very little relevance in the 21st century
"Public misunderstanding, ignorance and possibly contempt for liberty play into the hands of people who want to control our lives."
"One of the least-understood functions of private property rights is that of determining who may harm whom in what ways.""If we banned or restricted all activities that affect, harm or have the possibility of harming other people, it wouldn't be a very nice life."
Clearly not every kind of act that causes harm to others can be rightly prohibited.
Link50Dupes for the state
Link47Ban on Knives