Usually i charge and knock them over. Tried last night using steadfast ward and didnt work very well. Looking too what others do. Thanks

Arrows can knock them down, shouts or long range destruction may also. I try to approach them from the side and take the gem before it even casts its spell, but if that's not possible I just run in. Magic resistance is super handy.

Unrelenting Force works. I think that arrows do too.

Depending on the positioning, you can approach from an angle just outside it's line of sight, but this can be tricky when theres several of them or they're in an alcove.

Unrelenting force is good if you're not looking for stealth, but stuff does get scattered a bit.

Hitting them with an arrow is a good option if you have the skill.

Using a ward to protect you as you approach is a good option too for mages.

Fireball works for me if your character has it; bounces the gems right off the stand. If not and you can cast a rune spell from a sufficient distance and then set the rune off with a spell and that too will bounce the soul gems right out.

I use this after experimenting during the COW Questline, but it can be less Magicka intensive in most situations to just run up and just take the Gem ASAP and cast Healing after.