Film: At the house on Neibolt Street, the kids basically do a bunch of damage by impaling It with rods. It looks like they get these weapons from the rusty wrought-iron fence of the house itself. Down in the sewers, during their final encounter, it's much of the same. Mike brings the gun he uses to kill livestock, and the rest of the Losers come prepared with more rods. Beverly deals the final blow by putting one straight down It's throat.
Novel: In the book, the defeat of It is much more complex. This is where that werewolf comes into play. It becomes a werewolf for the second time when dealing with all the Losers. Part of the Losers' power is how much they believe in themselves. To prepare for this fight, they've made a silver slug. It, in this werewolf form, is especially susceptible to silver (like a werewolf would be powerless against a silver bullet). Beverly fires the slug with a slingshot and causes It to retreat.
Now, the defeat of It in the sewers is much, much weirder. As we mentioned before, this monster actually dwells on a different plane called the Macroverse. Here, Bill encounters a giant, ancient turtle named Maturin. Yep. Maturin actually created the entire known universe when he got a stomachache and vomited. This ancient turtle teaches Bill something called the Ritual of Chüd, which is a psychic battle of wit that can defeat the monster. So basically, in the book, Bill heeds the advice of the gigantic, wise turtle god, completes the Ritual of Chüd, and defeats It using the tongue twister that's meant to help his stutter: "He thrusts his fists against the post, but still insists he sees the ghost." And don't even get us started on what happens the second time around.
So, yeah. There's that.
