Hi,
The admin account is a requirement of the AEM platform. As a consequence, admin account cannot be deleted. Even if we accidentally delete this account, it will be re-created upon repository restart (in the default setup).
This is clearly mentioned at:
https://helpx.adobe.com/experience-manager/6-3/sites/administering/using/security.html
So what you can do is, you can simply limit access to the admin account by changing the password and ensure only the administrators of the system have the new value.
To change the admin user password, please refer:
https://helpx.adobe.com/experience-manager/6-3/sites/administering/using/security-configure-admin-password.html
https://helpx.adobe.com/experience-manager/kb/ChangeAdminPwd.html
We hope this helps!
Regards,
TechAspect Solutions