I'd delete the posts & users spamming in the blogs if I could mate, but unfortunately the moderator stuff isn't working as it should. Currently the only thing I seem to be able to do is click on a users name/avatar in a forum thread which pops up the mini-bio thing & a delete user button. If I delete someone then all of their posts & threads (that they created) get deleted with them.
There is no delete button for me on a persons actual profile page so I can't currently ban/delete anyone that way. Also for some reason I can no longer edit or delete other peoples posts. If I click on the edit button next to someone else's post I just end up going to an error this is not the page you were looking for screen.
As for your suggestion, it's a good idea - or maybe give the blog post author the option to allow or disable comments.
As for blog based spam I think a nice solution would be to prevent any user with less than x posts from being able to comment on blog posts or create them.