I've had good luck holding pulleys using a Harbor Freight chain wrench I bought for around $9. It looks like a vise-grip but with a length of chain on it that can be made into a loop. I use a piece of old v-belt so it doesn't cut into the pulley (or cam gear, etc. -- I've used it there, too, with great results, just used a piece of timing belt to protect the gear). You can then slide a length of pipe over the end and get a lot of leverage on the pulley.

1988 Isuzu Trooper LS
2.8l Isuzu 4JB1-TC intercooled turbo diesel
www.dieseltrooper.blogspot.com