Safety issue. unfriending someone still allows them to private message the member. Unable to prevent contact

We operate a private message by consent model whereby if someone wishes to private message someone they need to send a friend request which needs to be accepted by the other party. Only then can these two members send each other private messages. This works fine however, if a member decides they no longer wish to be contacted by this person and therefore unfriends them, the expected result is that they can no longer communicate privately and the person who has been unfriended is in effect blocked from private messaging that person from that moment on. This does not work as expected and although the first member has unfriended the second member, they can still send and receive private messages. Therefore there is no way for someone to unfriend someone and block them from private messaging them. This is not how it should work and not a safe option for a member who no longer wishes to be contacted by another member for whatever reason