How to prevent Gtk3 being installed?

Gtk3 sucks because the Gnome devs decided to break lots of GUI functionality that has been standard for the last 50 years.
The most annoying consequences are the scrollbar functionality being reduced down to unusability and the removal of window titles.

Thus it is very bad that many packages are built against Gtk3 by default.
This results in a general reduction of the usability of FreeBSD as desktop OS.
Worse even, this makes for an annoying UX for everybody who is no Gnome fanboy.

Of course the best way to resolve that issue would be the FreeBSD releases to be build against Gtk2 instead of Gtk3 by default.

So I ask what steps are necessary to completely get rid of Gtk3?

(If an application depends on Gtk3, I am not interested in using it, and would rather prefer it not be installed. If there is an installed app that requires Gtk3, it should just be removed if it cannot use an alternative library.)

To prevent Gtk3 apps from being installed and to remove those being installed, what would be the appropriate pkg delete command options?

After doing that, the Gtk3-dependent packages would then to have being rebuilt from source after configuring them to use Gtk2. (make sure you did make config before building!)

Would it be sufficient to modify /etc/make.conf as Remington suggested?
Much of this will depend on type of applications you need to run and how dependent you are on gui's. For example, are you OK with lpr printing, using motif based xpdf and gv? Do you want a window manager or a full desktop environment? What is your stance on Qt?

You can try to roll your own desktop with gtk2 but I suspect you will be sorting out dependencies for hours.
Using lpr is ok for me. I use FVWM but have KDE installed because there are some KDE apps I use regularly (mainly Kate, Konsole, Dolphin, Okular, K3b and a few more). Qt is no problem for me because it behaves well.
However, there are a few apps preconfigured to use Gtk3 by default. The most important of them is Firefox and the loss of scrollbar functionality is extremely annoying there, especially when displaying longer pages.

...but I suspect you will be sorting out dependencies for hours.
Exactly this I would like to avoid...
You can try and see what happens. I'm not too familiar with gtk3 as I far as I know it's buggy so I would suggest using gnome2, KDE or other windows managers.
More options:

Kate -> mousepad, leafpad, nedit, nano, vim-gtk2
Konsole -> urxvt, xterm with a tweaked .Xresources,
Okular -> xpdf, mupdf, gv
Dolphin -> xfe, mc (midnight commander)
K3B -> dvd+rw-tools (man page has copy/paste examples) cdrecord, xfburn
Multimedia -> command line mplayer
audio -> ncmpc/musicpd, madplay

I run xombrero and midori with gtk3 on some old hardware that is not powerful enough to run Firefox. My OpenBSD system with Firefox does not have a problem and I have good scrollbar functionality in FreeBSD xombrero/midori so I suspect this is a bug in FreeBSD's Firefox build.