if a member is banned he has to have committed a banable offense, everyone (including moderators) have to act within the rules set by the administrator, I do understand some members may start pointless threads which other members may dislike (even i may not like some threads) but we can not ban a member for that (unless he is selling something)
Its best to report the post and move on to some other thread as David has suggested, the mods will make an assessment of the reported post and act in accordance with the rules. Recently we received many complaints about profanity used by a senior member in a particular post who was expressing his views (rather strongly), he's been here a long time and usually doesn't respond that way so we edited out the profanity and mentioned it in the edit note. Issue was resolved and there was no trouble after that.
Yes forum promotion / advertising / spamming would definitely be banable and a serial offender who repeatedly violates a rules despite having been asked not to may possibly be banned but in other instances we need to evaluate on a case by case basis as I said we have to act within the rules,