As GM, clearly it is up to you to dictate what constitutes "part of a corpse". Maybe a tuft of hair is alright. One could reasonably argue that at least some amount of flesh is required, or at least some percentage of original body weight, and so on.
However I believe the real issue is not with the mechanics at all, but with the way the players are approaching the situation. You feel that they are somehow 'cheating the system'. Personally I think they're just getting into the spirit of the game. That they felt the need to plan ahead might be indicative of your GMing style. Why do they feel that way? Is the campaign excessively dangerous? That they did plan ahead should be rewarded. Trying to account for a more difficult campaign only to have their plans foiled will make for frustrated players.
Finally, there is certainly not a balance concern with this. Are you routinely dissolving your PCs in acid, or disintegrating them into dust? If they actually get annihilated to the point of no return, of course they are looking for smart ways to prevent it. I assume (and sincerely hope) that this is not the case, so really in the end it makes no meaningful difference.