Good Support Item Against Pantheon?

I play a lot of Nami, Lulu, Vel, Lux supports. I haven't played against many Pantheons, but I just went against one that got feed something like 23/1/10. I am a decent Nami player, but I realized that game I had no clue what item to even start building against the snowballing Panth (I think he had 3 kills in the first five minutes or something like that).

Obviously snowballing is one thing and most champs like Pantheon can't be stopped. I have a suspicion this particular panth was smurfing. I was also working with a sub par team that qued up as 5 from a previously well played ranked game, and the panth had the help of his jungle and mid. So in all the situation was unavoidable - they were just plain better.

But even if that was a "normal" situation, I have no clue what I should have built against him. I almost started Randuin's Omen but I never gained enough gold to get past my must haves.