It is summoned creatures that have no soul. Makes sense as their existence depends on Mana and not Life Force.
What svcks is filling up Grand Soul gems with common Drauger and Skeever souls.
This. However, you can remove trapped souls from gems. I read in a wiki, that if you drop the gems, and then pick them up, the souls are gone. I've tried it twice, and it works. You get a nice empty gem.
Also, the higher ranking Drauger, such as the death lords or whatever they're called, will resist your attempt to soul tap them. While you're doing that, he's putting the hurt on you lol. I just kill them now. Not worth getting beat up for.
There are enchantments you can use to trap a soul. I've seen them on swords and bows. Problem is, some require the enemy to be killed in less than 4 seconds - maybe hard to do unless you oneshot them. I've also seen 15 seconds, and that should be doable, especially on a high damage bow.
PS: This raises another issue. If there's a 4-second requirement in an enchantment, and you have that on a sword, for example, does that 4-second rule reset on each hit? If it resets on each hit, then there should be no problem with trapping that soul. Actually, same should be for a bow at 15 seconds. Does the second arrow reset the timer?