Setting a trap gun to blow away anyone who inserts a floppy (or hooks up a cable) to a machine he has not been given access to is morally permissable.
Morally permissable or not, a shotgun and a string are unlikely to be effective. The FBI went so far as to get a law passed that says that they can use classified technical techniques to execute the warrant so that they don't have to reveal their methods in court. Could those methods include something as simple as a backdoor in Windows, or some kind of hack into Windows? I don't have any evidence one way or the other, but it's a reasonable possibility. These guys are risk averse and they are on a budget, and sending in a team of armed hackers is both risky and expensive. Before you get the shotgun and some string and risk blowing your own head off, install a real OS. Do you think the FBI break-in team has an OpenBSD rootkit? Not likely! Then take some simple, non-violent measures to make your computer a little bit tamper resistant. It could be as trivial as a webcam pointing to your computer, storing its images off-site. Take a look at http://linux.davecentral.com/articles/view/1053/ for a neat program called motion which runs under Linux and works with plain old cheap webcams to do exactly this. My attitude is to think about simple things and think about ways to de-escalate a conflict as much as possible, so I'm not so enthusiastic about a shotgun on a string.