Ip bans + enable captcha and that is it, even if there are some that still get through you will still see the difference in posts and purge them out or just ban the member and leave the posts.
But i would i advise you to check carefully which posts you delete and which multiposter members you ban because some of them post really quality stuff and lately most of the posters do it threw the multiposter instead of manual posting and you might lose a lot of good members that way.
Try not to ban the multiposters that are posting in sections like music/movies/mobiles/tv-shows because not a lot of members post there and those sections are really important because we visit them frequently.