You need a HD box to show HD channels. HD deocders will down-convert to SD if you connect up your TV using SD connectors such as composite or S-video.
With most decoders, you can either edit the channel list and delete the excess channels (71, 72, 73 etc) or set up a favourites list of only the channels you want.
Favourites mode is probably easier and it'll only show the channels you select and if you want to add back a channel in the future you can just put it in the favourites list.
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