Comment # 1 on bug 964792 from Stefan Hundhammer

This is mutually exclusive. By design, the "taboo" status is only possible for
packages that are not installed. As for doing this as a convenience function,
this would introduce all kinds of really nasty dependency problems that would
be hard to understand - even harder than the normal ones - for the user. This
would work only for the most trivial of cases, and then only for really
advanced users.

Personally, I'd strictly advise against introducing this kind of behaviour.