When you render stencil shadows, you have to extrude the silhouette edges out a large distance from the light. If you only extrude them down to the first platform, though, they will only affect that platform. The shadow volume will also have to be capped for this to work, in case you're not already doing that.
By extruding the volume to a level between the top and bottom surfaces of the platform, you can also ensure that the shadow appears only on the top side of the platform.
Another possibility, if you only need to cast shadows onto flat surfaces (the platforms), would be to switch to projected shadows. That's where you render the shadow caster using a matrix that squashes its geometry onto the floor, and use blending to darken the floor in that area. You can use the stencil test to prevent double-blending. It's a good bit simpler than shadow volumes, if you can work within its restrictions.