More speartons.
But if you know the person is going to spear rush, and you're on a larger map, get 3 castle archers, all miner upgrades, statue health, 2 walls in front of the first 4 gold piles, and a mage with all your spells. You should be able to hold.

Personally, from the point of view of a noob spearton rusher, the hardest enemies when the rush has already begun would be the allbowtross. There's no way to attack them, and they dish out real bad damage to armoured units with their blazing bolts. This is just a rough estimate, but I'd say 6, maybe even 5 of them can two-shot a spearton. When you've got a giant or a shield wall there to stun, the allbowtross can tear right through most rushers. They're worse than the magikill; you can always dodge their poison, and you can even have a meric or two to heal poisoned units. But allbowtross? Dodging blazing bolts are impossible, there's no way to attack them, and they can kill your retreating merics as well, meaning you have to garrison and hope you have enough to start over.

Something like this is only a result of poor starting macro (on part of whoever is about to get spearton'd). However, there are soem good suggestions here, just not enough clarity on how to use them.

Yes, magikill are good on units in bunches. But it has a serious problem: If they have that many units, they can simple kill the magikill, and move on to whatever else. So you have to think a bit defensively, use walls, etc. Next is your own speartons, but against a smart player, if he has a starting macro advantage, by golly, he's gonna keep that. Meaning it's futile in the long run. So you have to mix it up. And that's where my crazy self supports use of flying crossbow men. I think it's quite intuitive for this problem.