Here's a sketch.

spanner wrench.jpg

I picked up the parts for fabricating this spanner wrench at my local Lowe's. For the studs I drilled and tapped the holes in the flat bar and locked them on with a nut. The stud bolts work well because their heat treated and won't bend easily. For the pivot point I used a bolt and nut loosely tightened.

The stud bolts should fit into the pulley slots. You'll have to position the wrench so it opposes the forces from tightening the pulley bolt.

I think this will work for you, but it's hard to say without knowing what your pulley looks like.

Have you got a picture?