'Micro adjust' saddle clamp not allowing fine enough adjustment (on SP12 suspension seatpost)
Although the damping effect of the seatpost is quite nice the clamping system dates from the 'medieval' period of cycling. Modern systems rely on friction between the surfaces. Instead of a silicone or rubber washer try with a thin aluminium shim that you may cut from a beer can. But take care that no sharp edges protrude from the sides.

If this doesn't work there isn't much else that you can do except bringing in the heavy machinery if you dare and remove at least part the of the splines with a Dremel or by filing them down. I'd do that on the blue bit with the fine file held in a vice and running the 'blue' clamping part over it.