Guild Wars 2

Just camp a spot with magic snow and once a player approaches and tries to build a snowman, hit him directly in the coupon.
While building the snowman, the player is considered wielding a snowball as well, so you can hit him and it counts for your achievement.

It also interrupts his building of the snowman.

Or..target him/her and let fly as soon as he/she finishes the animation. It is a bit kinder and considerate. =)

Wanze’s version sounds deliciously evil, think a lovely mob of people camping a single magic snow and repeatedly interrupting the poor soul who will try to build a snowman there >:D