Sure. Just un-join the RAW+JPEG thumbnails. You can do this by turning of RAW+JPEG mode (Command-J on Mac, Ctrl+J on Windows). Once they are separated, you can delete however you like.
I would also really like this ability. The problem with the suggestion you make is gets really messy with a large contact sheet, as you then would have to go through one-by-one selecting the RAW or JPEG version of each file you want to delete. The time-consuming bit is not the actual deleting, but the selection of only one part of a pair.
What I would like to be able to do is this:
1. Take my contact sheet of say 500 RAW+JPEGs (each viewed as single thumbnail)
2. Go through them and make my "bests" edit of say 50 pictures, tagging them all.
3. View only un-tagged, or view only tagged.
4. Select all
5. Then choose some nice new option to "Delete RAW, keep JPEG" or "Delete JPEG, keep RAW". This could be in a menu, contextual menu, or wherever really. It could I guess be in a dialog window arising from the general delete function, though I would think better to have it separate.
I can't see any easy way to apply the current tools to this situation.
Thanks,
Ben