[gentoo-user] Best way to block a given packages?

[Phill MV]

After a month worth of holidays, I return to my computer, sync, and type emerge -uva world.

Scrolling through the several screen's worth of stuff I find a few 'redundant' packages, like Mozilla and Epiphany (I use Firefox).
I use gnome, so I've always wounded up with these two packages on my system. All things considered, compiling these two is a waste of space, bandwidth, time and electricity.

How do I block these two from being compiled, despite the fact that they are listed as 'dependancies' in packages I use?