Is there a way to set things up so that the server sends no response to the prefetch request? Simply ignores it? It seems to me this would be better than feeding a 403 or a 404 or a redirect to a different page.
Sure, at least on a Linux box. This will hang the WA and the WA will have to give up after timing out. It will make the WA seem incompetent and/or broken:
This affects all domains on that box. The question is, will Google retaliate by assuming that these domains are also broken for Googlebot? As far as I can determine, Googlebot is on the 66.249.*.* Class B these days, so these blocks woudn't affect the Googlebot unless Google makes this assumption.
If you have a box where you don't care if you're listed in Google, it's worth a try. If you have a box where you depend on your Google referrals, I'd recommend a wait-and-see approach.