Perhaps you'll want to continue with the hGetLine setup in one thread (assuming the NNTP data is line delimited), then in another, parse the data, then in a third, respond.
Lookup how to use MVar's. Allowing the threads to block on reads/writes is a lot easier (logically) than figuring out the mess in a single threaded system. When you have a system like Haskell's threading tools, you're much better off splitting the tasks up into blocking calls with MVar's to synchronize.
(Perhaps MVar's aren't quite the correct solution here, but it seems like it would work to me.)