Board Thread:Warhammer 40k General Discussion/@comment-2253300-20130211000751/@comment-6078851-20130411005244

regarding the point about selectively erasing memories, that's exactly what is done.

the grey knights follow up as many chaos battles as possible by wiping the memories of every guardsman who now have overt knowledge of chaos (and if they can't be wiped they are simply killed). this isn't a choice. any who slip through the net or escape are relentlessly pursued by the grey knights, and any planet that these escapees come into contact with are subject to exterminatus, lest any knowledge is passed on.

the same rules apply to astartes, mostly. there are select chapters whom the grey knights allow to retain their knowledge but for the rest - memory wipes as standard. the biggest difference here is that astartes do get a choice - volunteer for the wipe or volunteer for execution - not much of a difference but the decision rests with the chapter masters and they consent to the wipes.