Unfollow is the right answer! Twitter is somewhere where there is no "forum" or "topic", you have to build your own out of people you find benefit from following. If someone's posting stuff you consider bad, then don't follow them. You control the quality of your twitter feed.

Big trending hashtags are always going to have a large proportion of garbage in. Report the spam and pick out the gems with RT; it's even possible to make friends this way.

I don't think it's actually appropriate to just decide what random people who haven't @-ed you and are talking among themselves is inappropriate. If it's one person harassing another, then sure step in and/or report. The big problem is when a celebrity turns their followers on someone.