Are there any plans to implement a "block user" function for the private messaging?
If so would it possible to make it so that when a user blocks someone that the blocked sender does not receive any notice that they are blocked?
"Shadow blocking" someone who is spamming/stalking/bullying, etc another user via PM is the best way to handle it. If the guilty party is aware (s)he is being blocked they will create a new account to continue harassing but if they are not aware and think the other party is just ignoring them they will give up eventually.

Please consider this feature for the Easy Social PM functionality as it will make a better experience for end users. It will also require for users to be able to manage their blocked users list so they can also choose to block/unblock users. Thank you.