In future in your offer letter itself please include a clause permanently " Please confirm your acceptance This offer letter is valid till___ and your failure to report to duty on or before ____ will lead to cancellation of this offer without any further communication. "
Now for the above Managerial candidate you may write as below
With further reference to our offer letter dated ____, you are supposed to report at our office on____. As you have failed to report at our office on____ and as there was no written communication from your end, we presume you are no longer interested to join us. Hence offer letter dated___ issued to you stands cancelled with immediate effect. Acknowledge receipt of this communication "
23rd August 2019 From India, Madras