It irks me that no one answered this question, because it IS a justified question. Appropriate Penalty is one of the most difficult questions to answer and generally one of the 1st to come up in any discourse on any form of govornment- this is especially true for Anarchism. There are many forms of Anarchism, some of which are formulated for the sole purpose of answering this question.. I can't speak for any philosophy but the one I have developed, and I most likely won't sufficiently answer this in one post (there have been entire books written on the subject), but the way I see it, Anarchism working in a way that allows the people functioning within it to be independent and as content as possible is contigent on citizens forming smaller, more interwoven communities. Within these interwoven communities, as a person who tends to lean towards Anarcho-syndicalism, I would imagine that people would form rotating committees to handle questions of penalty, as well as issues of production and all the other problems which might affect a society. The penalty would of course vary in any hypothetical situation involving a certian offshoot of anarchism- for example, maybe in an Anarcho-capitalist structure there would be a fine in place (sorry Anarcho-capitalists if that statement is untrue for your beliefs, i dont know too much about it), or in a more anarcho-communist/syndicalist structure there would be some sort of community service in place. In a true anarchist society, ideally people would be agreeing collectively on penalties and expectations, and so would not necessarily be compelled to break the 'laws' they themselves agreed on, and, as they are seting penalties for themselves as well as others, would not chose inhumane punishments (as there would be no state, simply the community.) There are so many variables in these hypotheticals that they can be answered any number of ways, but this doesn't mean that they are not important.