While I can certainly understand your frustration with somebody connecting to you for a couple of hours to download something, I can't understand why you share a file that you know somebody may take a couple of hours to download.
About the only way to limit this person would be to disconnect them and then possibly ban them from connecting to you, otherwise they'd just retry the connection, and upon getting through to you, you would want to disconnect them again. I do not believe that Gnotella has a "Minimum download speed" option, but I suppose that if there were one, there'd be a lot less to find on the Gnotella network.
If you don't want to risk the possibility of somebody downloading the file, don't share it. Of course, expect some people to disconnect you if you don't share enough, either.
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Take care,
Sir Eldren