Tools I've used to cut through silicone bonding items together have included razor blades, scalpels and fine piano wire.
The trick is to get a corner started and then continue through the adhesion zone with the piano wire, working it backwards and forwards. I always use new and high quality razor/scalpel blades.
I cannot stress enough the safety aspect of going down this path - there's a reason they use scalpels to open people up. Slow, steady and careful will get you there.
Resist the temptation to use force. Once, I tried to remove a siliconed Rule bilge pump base out by yanking on it. The layer of hull material that came out with it stunned me. Next time I promised myself to use the surgical method.
