If the spline on the shaft is straight and unworn, then possibly machining a new pulley would be the best solution. However, cutting a close sliding fit internal spline is a job for an expert with the correct equipment and know-how.
So what I would suggest is to purchase a new pulley, and see if the fit is improved just by replacement from a different batch of pulleys.
If that does not help, then perhaps you could make an anti-backlash pulley. Without seeing what space you have available, this could amount to cutting each pulley in half (by facing off half of the length) and bolting them back together with small bolts that lie parallel with the axis of the spline. If you make the holes slightly oversize in one half, then you can twist the spline hub a degree or so, thus reducing the clearance, and then tighten up the bolts to lock. When you have found a good setting that gives you quiet but smooth sliding action, then perhaps drill lengthwise through the hubs and pin the two halves of the hub together permanently.