its kinda simple. they have electrical components, but looking into it they are unfortunately waterproof [most of them]. I guess then you could kill them by using conventional means [stabbing, shooting, burning, blunt trauma, severing of the spinal connection, bleeding them out, blowing them up, etc, etc.] as im typing this i found out that prots cannot permanently survive in the vacuum of space and need oxygen to process nutrients and whatnot, so, choking or depriving them of oxygen or forcing them to inhale toxic gases is a viable option too. going back to the electrical part, if you wanna go ALL the way, you could throw large neodymium magnets at them to damage or dislodge their electrical components, and also stun and/or cause great agony/internal damage to them. better yet, you can [legally] obtain an electromagnetic pulse device, frying the prot's CPU and putting it out of commission.
or shoot it into space.
i'm not a psychopath. i promise. i know people say this to be emo or edgy or to get into cursedcomments but i want you to know that I am genuinely OK.
FORGOT TO ADD A FEW MORE: starvation/sleep deprivation and also deprivation of nutrients and giving them diseases and forcing them into poor living conditions.
tl;dr: kill them normally
