Project not completed and employee takes up another job - what should I do?

Dear team, this is karthikeyan from xxx softwares i am running a software company past 3 year. our company employee suddenly relived and asking for salary and experience letter he did not completed assigned project in given time. suddenly he said i have joined another company like that i asked about project completion he never cared about it what should i do give me a suggestion.

Dear Karthikeyan,
Have you issued the appointment letter to the employee in question? If yes, then what are the terms of the separation? What is the length of the notice period?
Issue the letter to the employee stating that he has to complete the notice period. Otherwise it will be considered as abandonment of the employment and appropriate disciplinary action will be initiated for the abandonment.
Thanks,
Dinesh Divekar