3 things can be done ;

1) Ask him to write a letter in disagreement to the warning to explain his version of why he shouldnt be issued a Warning Letter. The Warning Letter still sustains as 1st warning and is filed into his P-File.

2) Suspension ; The employee can be suspended from days to weeks, depending on HR policies.

3) Immediate termination ; If the warning is based on safety issues, and the employee refuses to acknowledge the issue, then termination can be in effect as a last resort.