I also dispelled their armor of agathys and countered their fireball when they changed up their tactics. Basically, if the warlock tries to cast, he gets countered and it really seems to hurt him far worse than a wizard or sorcerer. Has this come up for you much DMing warlocks or playing one?
My DM wasn't much into using counterspell. When he did try it out I instead cast where the counterspell-er couldn't see me (being a halfling helped) or out of its measly 60ft range.
Fireball has a cast range 2.5x longer than counterspell, though of course there isn't always room to take advantage of that.
P.S. how do you play the caster knowing which spells to counterspell? When do you announce it as DM?