Well... permanent mana barriers are basically the reason why you can't just walk around with a bunch of quickened spells indefinitely. You will have to re-cast and re-pay karma now and again, there's no avoiding that entirely

But, that's not helpful to your question, I know. So how about:
1) researching the barrier, and finding out where coverage gaps exist. "Hey, they warded the walls and the roof, but they never warded the basement or floor.... can we dig a tunnel under the foundation guys?"
2) the vampire exploit: trick the ward's creator into allowing you through. Hello, social side quest!
3) The "there's more than one way to knock down a wall" approach: in the case of a ward, finding and remove an "integral" part of the foundation causes a ritual to unravel. Pull the right brick out and a wall comes down! this is true physically as well as metaphysically.