Occasionally someone asks how to block a PM (private message) from another user. Most often this is the result of personal flame wars between individuals. Whatever the reason, if you just don't want to be on the receiving end of PMs from someone you either don't know, don't trust, or just don't like, the solution is pretty simple
Solution 1. If you receive an unwanted PM, just click the Block PMs from [sender's name] notice in red as shown here:
Once you click the above link, the sender's PMs will no longer appear in your inbox. And unless you tell them in some way, the sender will not know that you have them blocked. They just won't ever hear back from you, because you won't have their PMs to reply to.
Solution 2. If you want to be even more proactive and not wait around for an unwanted PM, you can add them to your block list preemptively.
Step 1. To get to your block list, click My Account, then People, then block list, as shown here:
Step 2. Enter the sender's name in the box and click Go and you will no longer receive PMs from them.