It depends on the extent of the bullying. Most bullying is insignificant enough to me personally that I don't think anything needs to be done. Only issues causing significant physical or mental harm are worth looking into. However, those issues are supposed to be noticed by teachers, administrators, parents, and peers. So we will assume that they are either not noticing or not taking appropriate action. For such things warnings and simple punishments are not significant enough motivators for this to stop so you either have to get the bullied child to fight back or take the bully and give them hell. As a rather sadistic individual myself I often took the position of observer and I noticed that in my school children that were bullied typically were part of multiple social groups, the bully group being a temporary group more often then their actual group of friends. The fix for this was entirely unnecessary because the bully group would rarely extend its influence to another group. Therefore most bullying I witnessed was insignificant, no solution necessary, and i feel the same can be said for most bullying.