I'm not sure if I should bother posting this, because I'm sure you've thought of it already, but just do a host check (leave it reasonible though), maybe have a max of ten descriptors available to any given IP, and if they already have 10 don't accept the connection. This might be a cpu-consuming task if you have a lot of players to check hosts on each time someone connects, but it should work until the kiddy finds another poor mud to attack.
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