Lv 100 is the final level for all pokémon in the series. Once it hits that level, that's it...
Magikarp can evolve into Gyrados starting at Lv 20 (as it's just a basic level-up evolution), but it requires leveling up to trigger the evolution, but there's ways to prevent it.
A) Place your Magikarp into Daycare -- Pokémon in daycare earn 1 XP per step. While you don't have control over the moves your pokémon may learn while in daycare, the workers will ensure your pokémon doesn't evolve.
B) Interrupt your Magikarp's evolution at each level up -- Trainers can always halt an evolution (by hitting B before the evolution is completed), but it's a one-time deal as the pokémon will try to evolve again at the next level.
C) Give your Magikarp an Everstone -- Everstones prevent the pokémon holding it from evolving at all.
Once a pokémon hits Lv 100, it can no longer evolve by leveling up (which includes friendship-based, held-item leveling & environmental evolutions). Other non-level based evolutions (trade, item-triggered & item-trade evolutions) can still be done.
While I'm a bit a skeptical you actually leveled up a Magikarp to Lv 100, as there's really no reason to do it. HOWEVER, it's not my business if other trainers do foolish things with their pokémon.