When employees ask for time to review the document and compose a rebuttal, provide them with up to seventy-two hours (or whatever period of time you deem reasonable) to return the warning to you. That seventy-two-hour window doesn’t provide them with any special job guarantees or protection periods. In other words, if you give an employee a written warning on Monday and ask her to return it to you by Wednesday, she could still be terminated on Tuesday if she commits an egregious act. What’s critical is that you, the employer, can state that the employee received the document on Monday, discussed it with management, and understood management’s expectations from that point forward.

On the other hand, if the employee refuses outright to sign anything,
you should understand that anger and a feeling of victimization are probably driving the person’s emotions. Explain that signing the document is important to show that he received it. Likewise, the manager should sign it to show that he is committed to both helping the individual succeed while holding him accountable for measurable performance improvement.

If the employee still refuses, encourage him to write a rebuttal. The employee gets the last word when it comes to progressive discipline:
Your assuring him that his rebuttal will not be rebutted makes for a more even playing field. Giving the employee a few days to provide his side of the story on paper is a good business practice because, from an evidentiary standpoint, a written rebuttal is as good as a signature.

Taken From : The Hiring and Firing Quention and Answer Book