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.
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In the Settings area, go to the General section and choose the Custom Fields tab from the left sidebar.
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The Custom Fields section will display a list of the existing custom fields on your site, both active and inactive.
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Hover over the field you want to delete, and click the edit pencil to the right of the field.
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Scroll to the bottom of the edit view, where you will see an option to Delete custom field.
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You will be presented with a confirmation modal. Click Delete to confirm and remove the custom field.
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For more information, see: Enabling or Disabling Custom Fields