Don't allow more than one upload, kill uploads at 0k. - There's probably no way to prevent 0k-uploads to happen, if you are on 56k. When somebody is downloading lots of files with a higher transfer rate, than his connection can handle, the slow downloads are throttled first, maybe even to 0k, - and that could be one of your problems.

I don't think, that ZoneAlarm, except for being completely useless, is a problem, if you are running LimeWire with server rights.