I doubt that. They'll just create more accounts to continue spamming.
I agree with that they can create new accounts for bumping but at the same time that is the only thing they will have.
About restrictions for new accounts... I don't think that normal user would notice it. There are certain patterns that shitposters follow.
Bitcointalk already tried to implement a "karma" system that was abandoned. The problem with a karma system that gives out negative merit is that whistleblowers are likely to accumulate a bunch of negative karma when they take on the perpetrators.
I understand your concern but that could be aimed specificly on the OP while the rest would be unaffected. So if I make this shitty thread and you all report it, moderator removes it and I would get negative activity or merit while the rest of you would just lose post count.
For about a year of my existence on this forum, only Vod and digaran's signature caught my attention.
This is the point if signature does not catch your attention it means that it is too saturated. It defeats the purpose of advertisment. But my main reason of removing sig from low ranks is to disencourage joining forum just for those campaigns.