Here's what's wrong with this suggestion:
members are supposed to post on topic, so theoretically there should not be a need for your flags.
Self categorisation is a ludicrously unworkable idea; and I'm positive the moderators would not thank you for the extra work involved in policing the extra layer of administration.
If you find a member that you feel persistently posts SPAM, then add that member to your Block List. The addition of an extra index, namely posting category, would require major surgery to the website. What do you do with all the posts that already exist?
How do you make the members use the correct category?
How do you know you don't want to read the on- (or off-, for that matter) -topic discussions?
[*]If the addition is costly (time and money), has additional costs in maintenance, has questionable methodology, has little possible uptake potential, and therefore has little point, why do you suppose someone in their right mind would want to do it?
[*]Why don't you build (or commission someone to build for you) a forum parser, that downloads a topic / forum / complete fora and adds all the categories for you and then allows you to search / sort / read by these categories? What's that? Too hard? Yes.