Simple: there is no such thing as an invulnerable system. I wish there were one too. See the 10 Immutable Rules of Security, especially the following:
Rule #3: If a bad guy has unrestricted physical access to your computer, it’s not your computer anymore
and Rule #10: Technology is not a panacea
You are looking for something that doesn’t exist.
You really need to do a comprehensive risk assessment and determine what risks you have and how much they are worth. Yes, I know. Who has time for that… Based on your findings from the risk assessment, do a cost/benefit ratio to determine if the vulnerability is worth the cost of the safeguard. Why would you spend half a million dollars on a robust security system on something that is only worth $1000 to replace?
Hopefully you start looking for the right direction: risk mitigation, not force fields. You should be able to find plenty of answers when you ask the right questions.
Hope this helps,