Blocking a conversation in a PM system

I'm making a PM system (private messages) on my website.

I want users to be able to block a conversation if the sender is spamming or being very unprofessional, for example.

For the user who blocks the conversation it will simply be blocked and removed from that user's list of messages.

Now what about the user who abused the PM system?

Should I allow that user to still see the blocked conversation, while blocking the ability to send more messages in it?

Or should the blocked conversation simply be removed for the spamming user too, without explanation?

Perhaps the blocked user would be provoked and contact the website asking "where is the conversation I had with that person?" or "why is my message blocked?". I would like to avoid that if possible.