Thanks for the bug report. Imbue Champion is an enchantment and is cancelable, and dispelling it does take away the target's ability to cast spells. I took a look at your save, and the problem is that Imbue Champion was cast multiple times on Gara, so that dispelling it through e.g. the enchantments tab on one of your units did not actually dispel the ones that were cast by other units in the same army. If you look at the enchantments ledger in the kingdom screen (which you can access by clicking on the mana entry in the resource display at the top of the main UI), you can see that the spell was cast multiple times.
It is a bug that it got cast multiple times, though. In the build you are playing, if you select an army on the main map and cast a spell without selecting a specific subunit in that army, every unit that is able to cast that spell does. This just leads to confusion, though, and bugs where spells that should only be cast once on a target are allowed to be cast on that target multiple times. In the next update, it has been fixed so that selecting the "cast spell" action when you have an army selected will be handled as if you are asking only the leader to cast the spell.
