I seem to remember that if you don't participate in the Snowtapult, then you wouldn't get hit, but that doesn't appear to be true since I have been hit several times since it started. Maybe that was only for random throws? I do seem to be targeted by the same person often...
I would like to opt out of this. Is there a way to do so?
I found when wearing I never saw the snowball event at all. As for opting out I really don't think there's an official way?
Unfortunately, there isn't an official way to "opt out."
Comes close but snowballs still make it through.

Is this for sure for sure gonna work? I mean, is that the way it's supposed to work?
Just want to be certain before I drop a cool 2.5 million. ;)
[Edit] Do you think it's worth it? Does it block most snowballs?
I dont think You can 'opt out' , but You can use the
Its in the MC for 2.5m ( Winter Section )
Ninja'd Sorry for ping. =/
It's served me well over the years... but it's up to you.

Full disclosure: I like getting hit by snowballs and think they're fun and am disappointed that they seldom show in my browser/OS combination or they just don't seem to stick.
If you feel you are being harrassed by a particular person you can block them. In the past when I used the chuck a random snowball feature, it would occasionally send me error messages "you cannot throw a snowball at this person because they have blocked you' So the Snowtapult will attempt to fire snowballs that it can't actually fling and the snowball remains in Inventory. If it still works that way this year then that is your inexpensive 100% solution to not getting beaned.
There's a chance that they are good-natured. If you like them or don't mind them in general you can ask them to stop. However sometimes in the spirit of the season or just plain cussedness this can backfire. In which case they have revealed to you IME that they are eminently blockable. You can make a note to un-block them come Spring, if you like.
I dislike blocking people unless I desperately need a break from them. I don't think this person is intentionally bothering me - it's probably just a convenience thing. Plus, I don't know them well enough to be able to say, "Hey, knock it off" without it getting awkward.
Thanks for the help! I'mma get myself all shielded soon. :)