First, I'm not sure I agree. Wardens use swords, guns, and even grenades. Would they really bther to learn the needed skills and do that so much if you could spellcast all day?
Still, there are a few House Rule ideas for that, my personal favorite is that you can cast spells at a number of shifts of Power equal to your Evocation Power Specialty in the element you're using without taking Mental Stress. This'll max out at one or two Shifts for starting PCs, but it should keep things from getting out of hand, which is important, since spellcasting is SCARY.