If you didn't sign it then just keep saying you didn't sign it, and refuse to sign it. Take it up with HR and/or with your boss' boss.
edit: idk why people are bringing up lawyers. You don't need a lawyer just for being written up or falsely being written up, just take it up the chain of command.
This is what you should do and if you're in a union, tell them you want your union rep present with you before you acknowledge anything. If you feel the claim is legitimately false, continue to refuse acknowledgement and a signature, do not admit to any wrong doing.
A lawyer will only get involved if you were terminated from your job under false pretenses and file for a wrongful termination suit. Save all evidence you have of the falsified documents and use a voice recorder app if you have discussions about it over the phone or in person.