Iirc fear makes them lose their COO control.Eh it's a problem to do this in RP, because you can't force someone to get flustered, they can just say it's not in their personality.
There are certain abilities (fear related, etc.) that can have techniques that limit Haki use, but just getting hit and losing some ability of COO is not a thing we can do, nor is it fully canon. It was a plot point in the Katakuri fight, it's not like people lose their COO control every time they get hit strongly. xD