There are some limitation on the forum software as it can't specified which sub-forum to be included in the new/recent posts for individual user (I'll see whether I can modified the forum code to include some similar feature), but one can still easily don't see the post they are not interested in.
Whenever i log in, i'll click on the 'New Post' link above, now you tell me how do i filter it off?
Please note that the page will also show which is the sub forum the new posts is in, so you just don't view any posts if it appear in the sub forum you are not interested at all...i do not think one can avoid sieving through posts that we are not interested to view, through the "Recent Topics" link on the header. what it does is to show everything that has been posted according to a time span. Unless I am missing something out...
Anyway, the posts in SingAstro isn't that much that we'll have 20 or more different topics daily, so everything should basically just shown in one page and anyone should be easily filter out those topics or sub forum that he is not interested in at a glance.
Please note that basically you'll only see multiple pages of new posts only when you didn't visit the site or didn't log in for a very long time.
Have a nice day.