Per host isolation will solve your problem as stated, but you'll still have the issue within the one computer that does the usenet download. So if you try to stream a video there, you'd maybe get 8/9 of the bandwidth going to the download and 1/9 going to your video stream.
In that case you can maybe cut the number of simultaneous connections to say 2 or 3 and avoid this problem, or you can try to tag those streams and use diffserv4 or something. You may also have bandwidth limiting settings on your downloader, you could request that it uses no more than say 20Mbps at any given time, or during certain hours or whatever, will take longer to download, but since you're probably doing long downloads anyway, it's not something you're necessarily waiting on so its all good.