Tamper-proof cap systems and methods for use with an access valve, such as a refrigeration access valve, are provided. A tamper-proof cap assembly for use with an access valve can include a cylindrical outer cap and a cylindrical inner cap, wherein the inner cap captively resides within and rotates freely within the outer cap. The inner cap of a tamper-proof cap assembly can threadingly receive the threaded end of an access valve. A tamper-proof cap system can include a tamper-proof cap assembly and a tool for manipulating the inner cap such that a user with the tool can manipulate the tamper-proof cap and screw it onto or off of an access valve. Without the tool the tamper-proof cap cannot be removed from the access valve.