If you are unable to delete a user, they are likely listed as the owner of the API key or owner of the integration. In both cases, you need to change the ownership first before you can delete the user. In this article, you will find instructions on how to do that.
The user is the owner of the API key
If, when attempting to remove a user, you were notified that they are the owner of an API key, you need to remove this API key and then create an API key for another active user. Subsequently, it will be possible to remove the user. How to generate a new API key is described in this article.
The user is the owner of the integration
Another common reason is that the user is listed as the owner of an active integration. A dialog box will display which specific integration it is. If, for example, the integration with Mailchimp is displayed in the window, go to Settings » Add-ons, open the specific integration, change the Owner field to another active user, and save the changes. Once the user is no longer the owner of any API keys or integrations, you can remove them from the system.