Prevent knockback when shield is active
Solution 1:
It's the drawback of the shield. All you can do is drop the shield and risk the damage.
Alternately, you can eat the knockbacks and spam weaker spells - Area spells are particularly good for this because they tend to not be channeled. I find that keeping a shield up and spamming arcane/earth/fire area spells can be quite effective against large numbers of softer hitting enemies that like to hit a lot of knockbacks (see: Goblins).