I'm not aware of any method short of removing the ability for the member to send the messages as covered in this thread.
Well, you're limited to one of two options. Get spammed by their Private Messages so you can read their posts. Or put them on ignore and see neither.
Erm...no. We're already hacked to death and unless the lot of you have been keeping quiet about something, this just isn't a popular tactic that annoying or trollish users use a lot. If they're spamming you, I suggest you follow the advice in the thread I linked to in Post #4.
Come on, people. Just delete it and be done with it. If it approaches harassment levels, contact one of the Admins and we will look into it.