Before the issue of the warning letter, have you given one-to-one feedback to the delinquent employee? Is there no improvement in him? If the feedback is not given, then I recommend you do it. There are several articles available on the web on how to give feedback. You may follow them. I have given an exhaustive reply on performance feedback. Though your case is different, many points of my reply are applicable in your case as well. "Warning letter" is negative discipline. Why invoke negative discipline if positive discipline works?
Ok...
Regards,
Dinesh V Divekar