D&D 5E - Warcaster Feat + Dissonant Whispers

It is a common, if not universal, ruling, that when you cast Dissonant Whispers at someone, and they fail their save, and they have an available Reaction, they *immediately* use that Reaction to turn and run away, which means that if they were engaged in melee with you, they're not Disengaging and they provoke an opportunity attack as they flee in terror. (Momentary terror; they do not gain the Frightened condition.)

Warcaster allows the use of an offensive spell as an Opportunity Attack, single target, converting its casting time from 1 Action to your Reaction.

Therefore, if you have Dissonant, check out Warcaster, and vice versa. If you have Dissonant and not Warcaster, then you can opportunistically stab them as they run, or whatever; that's already fun. The combination of 3d6 Psychic, plus making them disengage so you can cast Ranged spells without disadvantage, plus opportunistically blasting the target in the same turn with Scorching Ray or the like... yeah, that could be LOTS of fun, depending on what spell slots you're interested in expending.

What happens if, as they turn to run, you opportunistically cast Shocking Grasp, and hit, which then prevents them from taking reactions? Do they turn and start to run, get shocked, and then not go any further?