I contribute music and off-topic posts to several moderated fora. Moderation in itself does not guarantee off-topic posting. It all depends on what those who run the forum consider acceptable and the extent to which they allow the membership to run the forum.
In my experience the old 80/20 principle comes into play here - about 20% of the membership contribute 80% of the posts which leads to the feeling among that 20% of members that they should have the biggest say in how things are run - an oligarchy, if you like. That's the feeling they have but one of the reasons that they remain the most frequent posters is that newcomers are frightened off by a cliquey feeling and intolerance of difference.
I accept that it can be annoying to have a thread blown off course by humourous posting but if those who want to keep it serious have nothing else to add, then what's the problem? We can't MAKE others contribute in a certain way if they don't want to. If you have something else to say, keep saying it.
I don't think Mudcat's technology helps either. It's reassuringly antiquated - there are many more user-friendly bulletin boards that help you quote a previous post and get the thread back on track.