How to create an API only user

There is an option called "API only" below "API enabled" in profile. If you are looking in developer edition it isn't available but it's there in enterprise edition.

For those that do see it, if you are looking at a dev org created from an existing EE org, you should have it. However, if you are creating a brand new dev org that is not connected/related to an existing EE org, it is not there.

This is a permission not a special user. You can create a new API only user by setting permission at profile level:-

• Create a new User.
• Create a new Profile or clone existing one
• In new profile at Administrative section check the checkbox "Api Only User" and Save.
• You have created an API only user

When you check the API User checkbox, it restrict the permission of UI based login, you can't use credentials to actually log into the Salesforce instance. It count against API user license.