Just wondering if it's possible to disable the ability to capture a lesser soul inside of a gem that can hold a larger soul.

I've heard something even better: drop them and pick them up. Poof! No soul.
Mind I haven't tested it yet, would've help me a lot yesterday, had a lesser soul in a grand gem.

While that WOULD make a lot of people whine, I would actually be pretty glad if the only soul you could have in a gem is the one that's fit for it, OR if they stacked.
Too many wasted grand soul gems were sold with petty souls...

Yes, this is an issue where I would rather have empty Grand Soul gems than 20 of them filled with Mudcrab Souls.

One thing is you could give your soul gems to your follower until you are ready to use them. Might be a bit finicky to do the trade as you wade into battle with something you want to capture the soul of.

I have not heard of the drop a soul gem to clear out the soul trick. That actually should not happen.

The best way is to look at your inventory, and not to trap pety souls when grand soul gems are the only gems you have. Seems obvious, but some people seem confused.

If you want a real answer, just get Azura's/Black Star so you don't have to worry about that sort of thing. Or just leave all your empty grand gems at home until you decide to go out soul hunting.

I wish it would go from smallest to largest instead of the other way around.

keep plenty of empty common or lesser soul gems. I never have this problem cus i keep about twenty empty lower ones in my inventory.

I think this is how it works no?
Capture a lesser soul it fills the lesser gem in your inventory over filling the grand gem?

Make sure you carry lots of empty lesser gems then, soul trap always tries to find the lowest gem for the soul you absorb, so if you have no other empty gems besides grand in your inventory then they will get used.

One good way is just to kind of have an idea what kind of gems you have... and ... don't trap souls if you're out of petty or lesser gems. Sounds silly I know but it's crazy enough to work.

I am not a caster, I trap souls via my bow, it has a 1 second soul trap ability I put on it. If my little gems are full... I use my other bow that I actually enchanted to do damage. So I don't get the OP's problem, dont cast soul trap if your little gems are full or switch to a different weapon.

Skyrim is a little better that IF you have a smaller gem, it'll use that first. But it still mis-matches them, which really makes that perk in Conjuration even more useless. However, this SKSE plugin makes it just say it's "resisted" if there's no Soul Gem of *equal* size to put it in -- http://www.gamesas.com/index.php?/topic/1300172-rel-smart-souls/

And a lot of us on the PC are already using SKSE because it fixes the "Magical/Elemental Resistances are ignored" bug in Patch 1.2.

I think this is how it works no?
Capture a lesser soul it fills the lesser gem in your inventory over filling the grand gem?