It's only a moral dilemma if you look at the question in isolation from its consequences. If you consider the outcomes of banning an ideology, the argument is

It's only a moral dilemma if you look at the question in isolation from its consequences. If you consider the outcomes of banning an ideology, the argument is quite clear and rational - and not hypothetical - this experiment has already been run, many hundreds of times. In (very) short, if you limit the spectrum of political thought you achieve two major things: 1) you push the hard elements of that ideology underground. There are no good outcomes from or for an embittered, isolated group of people within a society that spurns them. 2) you eliminate an oppositional force, thus shifting the pole of political thought, and end up with a previously benign set of ideologies mutating away from previously nuanced stances towards more totalitarian approaches. In the absence of traditional reactionary forces and diametrically opposed thought, new axises are found. Essentially, once you commit to a single ban on ideological grounds you are committed to make further bans, as concept creep occurs, and new categories are found unacceptable. This occurs geometrically, and you fairly rapidly end up sending people to gulags and building roads of bones. It is impossible to go back without capitulation of government, as your own thought police will turn on you, and your eager acolytes will happily fill your shoes. These states end up ending in the fire of war or the ice of economic collapse - there's very rarely a happy exit. You can ban things, in the form of prohibition, at the expense of enabling organised crime and black markets, but the moment you start banning ideas, you are truly damned. If there's a historical precedent where there's been a good outcome from banning thought and speech I'm all ears, but I'm aware of none.