Preventing installation of specific packages

[Jason Heeris <jason.heeris@gmail.com>]

I'm creating a fairly simple rescue USB drive, tailored towards a
specific low-end machine I'm working on. I'd like to be able to
prevent any MTA from being installed (amongst others). Since it's only
a recommended package this should be possible without breaking things,
so what is the simplest way to do this (without completely disabling
all recommends)?

On a more general note, how can I select a specific recommended
package over another (eg. anacron instead of cron)?

Cheers,
Jason