As the snow ant is yet another member of Team Ant, the techniques for disposing of said ants mentioned there also apply to the snow ant. In other words, make judicious use of Elbereth, attack wands and ranged attacks, try to fight them in a corridor or (preferably) near the up staircase, and don't be afraid to run away or use escape items. Note that their cold attack is able to destroy potions, but not scrolls. This attack is affected by magic cancellation.