yes, but it's a bit hidden. I've never known why!
Go to "My Messages", then go to "Preferences" and "Manage Rules". (Or alternatively,
just click here)
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Click on "Add new rule" in the bottom right
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Fill out this form like this, but change
"the_spammers_name" to the username of the guy you want to block
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Check the description in the bottom, does it say what you want it to do?
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Just hit save rule:
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This should delete all the messages coming from the username you supplied. I haven't tested if this works, but I imagine it does!