Dear Mr JP Srivastava,
There is no need to write a separate letter on abstention from the training. When you issue a notice or circular on the training programme, add one more paragraph stating that the absence from the training will be construed misconduct suitable disciplinary action could be initiated against the absentees. I recommend merging advisory note in the main circular itself to avoid increase in correspondence and thereby your work too.
I do not know your designation. However, get the circular signed by higher-level authority like VP, Director, MD etc. It lends the weight to the circular.
Thanks,
Dinesh Divekar