You can't delete something from the blockchain, once it is posted, it's on there forever. But it is a valid question, why they haven't implemented a delete by "marking" a post as deleted and then just not displaying it anymore..
What you can do right now is edit a post and it will show the updated text. That's the reason you are seeing so many posts with a "delete" placeholder text.
!gif delete
@unitqm Actually you can delete a post (at least at https://ecency.com/). But I guess in the background they are actually doing what I wrote in my previous comment:
Via Tenor
how do you insert a gif here on hive in post or comment?