Kalypso, what you are doing should work from a pz2 (pose) file, but it's easy to get the syntax wrong or make a typo which stops the file from having an effect. We need to be clear what we are trying to do here. In Poser library files 'bend' has a technical meaning which is not necessarily what you might at first think. In Poser script 'bend' means that when a joint is rotated the "skin" round the joint stretches in an organic way. Here is an example of a pz2 to turn the 'bend' property off:

{
version
    {
    number 
    }

actor abdomen
    {
        bend 0
    }
actor head
    {
        bend 0
    }
}

On the other hand in Poser script what we usually mean by "bend" in every day language is actually 'rotate?', where "?" is a place holder for X, Y, or Z. The easiest way to stop an actor from rotating is to set 'locked 1', the 'locked' line usually comes at or near the end of the actor block. A pz2 to implment this might look like so: { version { number }

actor abdomen
    {
        locked 1
    }
actor head
    {
        locked 1
    }
}

The problem with using 'locked 1' is that all the paramiters for that actor will be locked, target geom (morphs), scale, translate, the works, not just the rotations. Most of the time this will not matter, but if it does, the only alternative I can think of is to set the limits, but if you set the limits you will have to specify min and max values (which should be the same) in the pz2. In other words you have to specify the amount of rotation in the pz2, there is no way (that I can think of) to use a pose file to set limits for the current values of rotation as they exist in the doc.