Can you repulse Call the Weak?

No, you cannot use Repulse to counter Cull the Weak.

According to RiftJudge FAQ #10429 and FAQ #8598, this is because Cull the Weak does not target.

Reasoning:

1. Repulse's Requirement: Repulse is a [Reaction] that specifically requires you to "Counter an enemy spell or ability that chooses [a friendly unit]."
2. Cull the Weak's Mechanics: Cull the Weak states, "Each player kills one of their units." This instruction does not target or choose any unit when the spell is played. Instead, each player performs the choice of which unit to kill during the resolution of the spell.
3. Conclusion: Because Cull the Weak does not choose a unit as a target when it is placed on the chain, it fails to meet the fundamental requirement for being countered by Repulse.

This is an interpretation based on the mechanics of non-targeted effects; no official rule explicitly lists every spell interaction, but the FAQ entries for both Repulse and Not So Fast consistently confirm that non-targeting spells cannot be countered by cards that require a target.