Well, you can... just not with SVN...

What I'm talking about is within a company, where there is some amount of central control. A
poison pill that the poisonee has to agree to is *not* pointless, if it is processed automatically
and allows the user to not care that it happened. The agreement would happen ahead-of-time,
when the clients are setup to trust the central repository.

Of course it's not possible to (remotely or otherwise) destroy all traces of information. Your
mistake is jumping from that true statement to the suggestion that it's pointless to do anything
at all. And that's simply not a supportable jump. Yes, an Evil User could intentionally save a copy
and Do Evil, but in a corporate environment where you hope you can trust everybody, it's often
quite sufficient to simply avoid explicitly tempting people to do something bad.

At the *very least*, I need to stop distributing the data (that is: remove it from the central
repository). And just that alone will cause significant user pain if everyone's using git. Everyone's
branches and working copies need manual intervention to clean up. Not a fun state to be in.
Contrast with SVN, which implicitly trusts the central repository, and just goes with the flow...