Jonathan Pie did make a good point that banning Trump could end up doing more harm than good. Basically, it'd legitimize him in the eyes of many of the undecided and outright lionize him in the eye of his fans, galvanizing them and making them even more likely to commit terrorist acts. It confirms their fears and makes him a living martyr to their cause.
Of course, its a serious balancing act between keeping these dumb monkeys where we can see 'em and not allowing people to incite violence on a platform as public and accessible as Twitter.