This is an idea i've been toying around with for a while now, and i'd like to discuss how it could be done. I know this is treading into "special snowflake" territory, but with the unpedictability of Chaos compounded by the Just As Planned of Tzeench, i figured such things must be possible. the Way I see it, there are a few different ways this might work.

1) Ahriman, found a way to reverse the Rubric without also causing massive instability in the Thousand Son's Geneseed. this particular Marine is a product of such experimentation. Mechanics wise, I think making such a character forever barred from being a psyker or gaining any kind of Gift of the Gods makes sense.

2) the neither reversed the Mutation in this Marine, nor did it turn him into a mindless husk of armour. it just halted any further mutation. a marine like this would still have all kinds of ridiculous mutations and be prone to suffer further mutations (give him the Humans mutation track when gaining corruption). he just isn't Mutating out of control

3) the character IS a psyker, but his psychic power is turned toward making sure that he doesn't mutate like nobody's business (except maybe Tzeench's), so he doesnt manifest any outward powers, instead being intrinsically resistant to most powers and perhaps possession.

what do you guys think?

Well, I mean, it's not canon, but if it works in your game I say run with it. Chaos can do some weird **** sometimes.

Easiest way to explain it, I think, is just have them be a late-millenia recruit, a non-psyker who received his implants well after the rubric of ahriman.

another avenue is a psyker whose rejected his gifts. so they're still a psyker, and they just REFUSE to use their powers.

I could see as 10k sons sorc trying to walk away from his pwoers after the rubic was cast sort of a "omg what have we done, I'm rejecting this"

not sure if this will help or not but in one of the HH series books, can't remember the title but it was the something dead, a psyker cuts off his connection to the warp on purpose to escape a deamon. your character could have done something similar.