Sounds like a possible (but less severe in this instance) devnull attack to me. There was a stink about this not too long ago. In fact, it's capable of doing attacks so severe that it caused a GMod server admin to get a $15K bill. The thread about it on Facepunch appears to have been deleted, though.
Try getting the IP address(es) blocked. I'll let someone else cover any srcds.exe tweaks if there are any.
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