What are the different ways to set an account to inactive?

You can inactivate accounts and contacts using:

• The Sales Accounts UI: In the Edit Account or details screen, delete the account.
• REST APIs: See How do I delete a record and its child entities using REST API? and Delete an account
• SOAP web services: See SOAP Web Services for Sales and Fusion Service
• Import Management: See Understanding Import and Export Management for Sales and Fusion Service