Disable a user is an important measure to be taken when an employee leaves the company or when they must not have access to the system anymore. Unlike locking a user, which is a temporary way of disallowing a user to log in, disabling a user is a permanent action. However, it is not a destructive process and it can be reverted if needed.
The only way to get a user account disabled is by asking the support team to do that. Open a support ticket and specify which accounts must be disabled. If you know that a user is accountable, please indicate which user will replace it. With this information, the support team will help you.
Please refer to this internal article to see the steps to disable a user.