I am new to steemit, don't know the algorithm yet. But the trick is to limit the number of upvotes per day and increase the number based on trust created with time and again decrease it with distrust created with time and also bring downvotes (though less stringent, and limited in number per day) for comments and posts. We need a bit complicated algorithm. For example, when you are downvoting, you need to specify complete reasoning about the downvote. In this manner irrelevant post will be weeded out.