Nothing ever gets deleted on the blockchain. Once a transaction(upvote/comment/resteem) has already been done, that same transaction will stay on the blockchain for eternity.
However, what most Steem Platforms like Steemit, Busy.org or eSteem does, is hide the post visually or in UI. A "delete" button is then seen beside the "Edit" button if a post/comment didn't receive any (Zero) upvote/comment. If a post receive even a single upvote, the "delete" button is gone and only the "edit" button is visible.
In addition to this, there's also a time limit before the Delete button is gone. I'm not sure exactly how long it is though.