Pretty sure this isn't the most elegant way to do it, but setting the /Volumes directory immutable prevents volumes from auto-mounting:

% sudo chflags uchg /Volumes

Then to revert back:

% sudo chflags nouchg /Volumes% echo The Bird is The Word

Despite being an OS X user since the first 10.0 betas, I know surprisingly little about how Macs handle volumes. Put me in front of a [purely!] FreeBSD, NetBSD or Linux box and I'm set, but Mac has its own way of doing things. Of course ;).