ActivityPub is similar to email in a lot of ways but has some key differences. One of them is it's based around an "activity" feed where users can initially "create" posts/comments/etc. It also supports other activities though such as "update" (edits) or "delete" which are propagated across instances which have the content.
It should be that you can post a delete activity and the origional object (comment/post) is replaced with what ActivityPub calls a "Tombstone" which is basically just a place holder, this delete activity should be then federated across to other instances.