I have noticed a number of blatantly off-topic questions that hang around for a long time despite heavy down-voting and calls for removal by senior users. I found somewhere on the site (although I can't find it now) that 10 flags would remove a post immediately, so I dutifully add my flag; but flags do not seem to accumulate enough to get rid of the post in good time. The problem may be that, while it takes only a couple of seconds to identify such a post with certainty, it takes much longer to flag it (including typing out 10 characters such as off-topic-, which seems unnecessary since the off-topicality can be seen at a glance). Also, people may not be aware of the 10-flag rule.
I am only talking about obviously off-topic questions, which genuine users would unanimously want to remove, such as school homework problems and muddled questions by people who don't even understand school maths. It seems that they are protected by well-intentioned procedures aimed at giving some benefit of the doubt to questions of borderline topicality (which are not much of a problem in my view). In contrast, posts by trolls and spammers seem to be very well filtered.