Within Teamwork Desk, you can delete a custom field to remove it from your site entirely.
Click your profile icon in the top right of your site and select Settings from the dropdown menu.
In the Settings area, select the Company tab and then choose the Custom Fields option from the left sidebar.
The Custom Fields section will display a list of the existing custom fields on your site, both active and inactive. Click the relevant field to expand the corresponding edit options.
Scroll to the bottom of the expanded custom field area, where you will see an option to Delete Field.
You will then be shown a confirmation modal before proceeding to delete the custom field.
For more information, see: Enabling or Disabling Custom Fields