The only way to remove the electromagnetic fields of the computer mouse is to create a closed metal box around the internal circuits.
The best solution would be to have the entire mouse shell made of metal but this would need a mold for fused metal which would cost too much, unless the product is ordered in a large quantity.
We have tried to wrap the mouse completely with aluminum foil, leaving just a small hole for the optical sensor and another small hole for the wire.
It is also possible to shield the mouse circuits with aluminum foil inside the mouse cover, but in this way there are more holes in the shielding and some EMF actually escapes out.
The results with an external shield are very good, the pain in the palm and the fingers have disappeared.
The following are some pictures of a shielded computer mouse used for testing. The external aluminum foil has enough shielding effect, but it breaks down after some time.
Our current best solution has copper foil inside the mouse cover, it is much more robust and externally is identical to the original, you can see the current solution here.