Rather than jumping directly to the warning letter, I recommend counseling him once or twice. Keep records of your counseling. Issue a warning letter only if he does not improve after counseling.
Counseling Authority and Competence
How senior are you? I ask this question because counseling has to be done by someone senior. Just because you are from HR, your function does not give you the authority to counsel. Secondly, competence in counseling also matters.
Yes, there is nothing wrong with issuing warning letters as such. However, it would be tantamount to using a hammer to affix a pin to a few papers! Always keep in mind the principle of progressive discipline.
In case he receives a warning letter without counseling, he may be put off mentally. It is not in the interest of the organization to have any mentally disconnected employee.
Analyzing the Employee's Behavior
By the way, why is he talking too much? How does he have time for this activity? Have you done a workload analysis of his job? Does he have sufficient workload? Increase his workload. Secondly, explore the possibility of sending him out of the office for some fieldwork.
If he is talking too much, then he could be committing errors in his job as well. How is his quality of work? While counseling, in addition to garrulity, include this point of "inferior work quality" if feasible.
Regards,
Dinesh V Divekar