Addressing Employee Non-Performance
As mentioned by you, the marketing executive in your company is not responding when asked to meet the client. You can definitely take strict action against him, starting from issuing a warning letter to termination. However, I would suggest certain points to be considered before you take any action against him.
Has he been behaving like this ever since he joined your organization?
Has there been any confrontation, discussion, or counseling done for him?
Has there been any discussion around the reason for this behavior to analyze if this is a skill issue or a will issue?
My questions may not sound very important, but if he has shown good performance in the past, he can do so in the future as well. As an easy alternative, you may fire this employee and hire a new one to replace him. However, if any 'performance improvement plan' in your company can make changes to his performance, you'll convert a non-performing asset into a valuable one for your company! Moreover, it can help the retention figures of your company look good.
It's a fact that the employee in question is not performing, but the REASON is something that needs to be taken care of. Just a possibility, he might be facing personal problems, frictions with his boss or colleagues, or any other problem inside or outside the organization. This given situation should ideally lead to an investigation or diagnosis of the problem, which may bring some unknown facts to light. It could be a mean statement, but such situations are helpful for HR personnel to engage themselves in understanding the psychology of the employee, his understanding of his own work, and his views towards the employer.
Undoubtedly, the situation calls for strong action, but whatever action you take should be supported by reason. At the end of the day, it's a 'REASON' that can justify the actions taken by management or an employee to survive and progress professionally.