To add another 0.02, I'd use a pair of pliers to apply a small twist deformation to the rim of the can (at some position that won't touch the mouth or the nose)
This can be easily felt, cannot be removed (like stickers, rubber bands etc), manipulated by a guest (an ocd type who will reset all the rotated ring pulls), is quicker to apply than other manipulation resistant methods (tying string) mentioned and is more discreet (no one will take notice of can damage, but they might mess with a rubber band)
I'm not sure whether I'd mark the sugary cans or the non sugary ones; it's for you to decide I guess. This method can't be undone by accident and I find it likely that a blind person would be able to feel the difference between a can that had been twisted with pliers versus one that had a damaged rim from being dropped so really you're asking whether it's better to mark the sugary ones because if you miss one then you have a sugary drink that will be ignored (isn't a hyperglycaemic bomb) or mark the diet ones because they're low in number and you're highly unlikely to mark a sugary one. The fail safe will be based on your confidence in the accuracy with which you can complete your task of marking the cans plus how likely you think accidental damage could be mistaken (if you mark all the sugary cans and someone damages a non sugary in the exact same way then you risk hypoglycaemia instead of hyperglycaemia; it's easier to treat hypo than hyper because sugar is more widely available than insulin). Personally I think I'd mark the sugar ones.
ps; I say pliers but actually any device with a slot in it just wider than the width of the can (think of a very small spanner) could apply a twist. Additionally your fingernail, back of a knife etc would be able to make a repeated crease in the shoulder of the can (where the can widens out after the rim then transitions to a vertical cylinder) that could be felt, and cans would also be quite likely to survive the rim or base (where it contacts the desk) being dented with a pattern (such as three in a row) that could be applied by any sharp, hard object such as car keys or even the rim of another can (thus marking two at the same time)
In this latter case of getting more adventurous with deforming the can, maybe make it the sugar free ones you damage, as it's a lot less sticky if you have to clean up after bursting one. They take a lot of punishment in transit though so I think you'll be fine if careful