Provide an active three long-range plan to deter and deny external perpetrators access to the corporate center?

Per Fennelly (2017: 219-228), countermeasures concentrate extensively on deterring unauthorized users entering the organization’s security center. It is worth noting that external and internal users are conspirators. External perpetrators often gain legitimate access to organization’s network center in the guise of technicians, professional consultants and irritated employees. You have been retained as the lead director of Adam Smith, Inc. Security Data Center:

• Provide an active three long-range plan to deter and deny external perpetrators access to the corporate center?
• Provide an active three long-range plan to deter and deny internal perpetrators access to the corporate center? 
• Provide an active three long-range plan to deter and deny disgruntled employees access to the corporate center?
• How can you identify such individuals or group of individuals?