bob marley - stir it up

so i was looking at anarchism tonight and the little i read about it i got this 1) it is all about not having anyone able to force anyone else to do anything 2) mutual agreement and cooperation is how things get done .... so i came to two problems ( i am trying to remember them at the moment) one is what do you do about people that do something wrong (say kill another person) because you can't just let them do that and it also goes against the 1st rule if punish them.  it was discussed that the future of those people would be determined by a jury of people or some third party ya know... which is in effect having someone force another person to do something so basically the whole anarchy thing contradicts itself in its simplest form but obviously nothing works in its simplest form so lets suppose on that one situation you have a group of people determing the "offenders" future... then who determines what grants an "offense" cause abortion for example would be something that a mutual agreement among a lare group of people could not be reached, which in effect would lead to the need for a smaller group of people deciding how everyone else will be allowed to behave (abort the child or not).... so perhaps there are some anarchist out there with solutions to this?