How do you counter Warlock?

Besides going for Diffusal Blade, which was already mentioned, you also have a number of other strategies to diminish Warlock's effectiveness:

1. Play around Chaotic Offering's cooldown. Warlock is quite unimpressive when his ultimate isn't ready, and the fact that he invested ~9000 gold for the quadruple Infernal doesn't help either (means no Shiva's / Hex / Mek / Force Staff to help in smaller skirmishes or when the ultimate is just cooling down). If you can force him to blow his ultimate when he doesn't want to (for example to save himself or his teammates from a gank) you don't have to worry about it for a very long time, thus you can punish him by taking towers / Roshan or just generally playing aggressively without fear of being punished.

2. Prevent him from getting a good ultimate off. Chaotic Offering works best against clumped heroes in a teamfight, when you can stun them all and shred them with those Flaming Fist procs. If your team just spreads out and forces the enemy team to engage in smaller fights and / or to defend from splitpush, he will be forced to either blow his ult prematurely (and you can punish accordingly for that, see 1.) or to not be very helpful, which can lead to towers or heroes down.

3. This is quite a general advice, but just killing him repeatedly works very well. Warlock is very susceptible to ganks early (no burst damage / heal is too slow for it to actually make a difference), and is still relatively easy to kill even in mid / late game. Good blink initiators like Puck / Shaker should give your team enough time to just get rid of Warlock before it casts his ultimate, and even assassin heroes like Orchid Clinkz / Storm / similar heroes can just remove him from the fight if the opposing team isn't on point with detection / counter initiation. The fact that Aghs / Refresher don't give that much in terms of defensive capabilities (compared to Mek / Force / Shiva / Guinsoo) means it's even easier to remove from the fight before Chaotic Offering comes out.