On the non-drive side crankarm there should be a cap over the 8mm bolt (It's a 10mm hex) so when you loosen the 8mm bolt it presses against the cap and pulls the crankarm off the bottom bracket. Given that you could remove the 8mm bolt, it seems the outer cap is missing. Your lbs should have a replacement or have a cap they can use to pull the crankarm for you - in my shop we have all such caps for their respective crankarms hanging on a hook along side other pullers. Loosing these caps is a fairly common occurrence.