Dear rmy_nbg9078,
According to law you cannot force a worker to work overtime.If you are planning to give a employee a warning letter what would be the basis of it and what would be content of it as it would not be valid in labour court.However, performance can be made basis for issuing of warning letter.But, i would request you to kindly have a word with him and try to counsel him/her that this would be important for his/her performance appraisal and it is very important for his and the organization's growth.
As regards to your second query.Yes, an employee has a right to refuse to sign warning letter if he does not agree to the contents of it.
However, you can keep a copy & orginal in his personal file mentioning on the warning letter that the individual has refused to accept the same .
Regards
C.M.Mohla