If there is, then throughout several playthroughs of this game, I wasn't able to find it. However while it is occassionally useful (especially if you've guardianed a tough unit to a mana font that is no longer being contended), I only found myself really wanting to do this a few times. More often I found the easiest way to respec my forces was to throw them at the enemy (except on the last level - do not
do that on the last level).
Actually, I found an answer. You have to go to the menu and add new keyboard shortcut with "drop a soul". If you'll press it, it will cause leaving blue soul on the ground.