When using the "report/block" link on a message, you will be taken to a confirmation screen.  If you supply a reason you are blocking them and click the "Confirm" button, the following will happen:

The reported/blocked user is blocked from sending you rMail ever again.

You are blocked from sending them rMail ever again.     Their "abuse count" is incremented by one.

The user is notified that you have reported them.  Note that your RPoL Account Name is revealed by this action.

The rMail you are reporting is copied -- in full -- to site Moderators, who will investigate the issue.

If the abuse report is found to be false, the original penalty is reversed, but the reporter's abuse counter will be incremented by two.

Long story short: only report/block those who are actually abusing the system, and never do it lightly or to get the final word in a disagreement. 

That said, we strongly recommend that people do report unsolicited adverts and unwarranted abusive content.  rMail is largely policed by the users, and proactive management of inappropriate use of rMail will reduce the occurrence of unwanted rMail for yourself and all other users.

Do note that RPoL Moderators and the Site Administrator cannot be blocked from sending you rMail.