When people reply to a topic. Sometimes there are so many replies and to be efficient users would like to hide the less important ones. How can this be done and make it stick through refreshes and logoff and logons?
Unfortunately that is not possible in bbPress and to be honest I have not seen any plugin doing that so far. There are some discussions ongoing concerning bumping and sinking where replies could go higher or lower in the list, but that discussion is still ongoing and not so easy to implement.
If you want to program yourself, I would think about adding the ID of the replies you are hiding to the meta of a user, or adding the ID of the user as a meta of the reply and then based on that display the reply or not.