The mechanisms are plenty: @cheetah, then there is a bot called Steemitwatch or so. Then there are users who hunt people abusing the platoform and write posts about it. Then whales often come and downvote that content. I don't know much about these mechanics in detail but since everything is transparent and public nobody can hide what they do. They try and it is sometimes difficult to discover but eventually somebody finds out and does something about it.