http://facepunch.com/showthread.php?t=1386818
If your clients are complaining about "*cough*" spam or similar through Steam chat, this is what's causing it. It may be wise to just completely kill your server until the Facepunch folks release an update to fix this (even though it's 3 AM in the UK), as it's rumored that even if rcon/client uploads are disabled, it'll still work.
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