Due to domain name regulations, once a domain is registered, it will remain active until its expiration date. This is the reason why active domain names cannot be removed or deleted from your account, and also for domains not being refundable.
If you have a domain name and no longer wish to use it, you can simply let it expire. You can check when a domain you purchased with us expires by going to the Domains section of your panel:
Once it reaches the expiration date, the domain will then enter a phase where it may still be renewed - and after that, it will be deleted automatically. You can find out more about the domain life cycle here: What happens when a domain expires?
In addition, if you were using the domain with your hosting plan and wish to remove it from the hosting, you can follow these steps: How to delete a website from a hosting account? This will remove all files, databases and emails associated with the domain from your hosting.