Regarding your proposal, I would suggest that signatures be automatically turned off on all posts after 5 pages, because that is more than enough to discuss something constructively, and everything after that is just a repetition of the same ideas and attitudes.
That measure would only reduce the amount of posts written in those threads and nothing else, as spammers would easily find the sub-5 pages topics and continue to churn out their spam or they would create new ones. Maybe its just me being used how other forums operate (mods there are less tolerant of duplicate threads) but imho bitcointalk already has way too many topics being opened every single day and any measure that would cause even bigger amount of those is bad in my opinion. Beside that, it would be unfair to penalize posters some topics in gambling board that naturally have a lot of pages.