This FAQ will help you how to delete a lead through API. You can delete a lead through API internally or a 3rd party lead vendor can delete the lead using Request Query String.
(Note: First is you need to add Authorization Token, click here to see complete details)
How to delete lead internally:
Note: You must know the Lead ID of the lead that you would like to delete.
Sample Lead to delete:
From the Admin page, click on API and select Leads.
From the Leads API, click on Delete Lead tab.
Check the Arguments when deleting leads.
Click on Test Request tab.
Enter the Lead ID 19584200 on the lead_id field and click on Run button.
Select the correct Authorization Token for this API from the dropdown box. Click on Run button to execute the action.
Below you will be able to see the system's response.
Search the lead ID 19584200 (No results found)
How to delete lead by your lead vendor through API:
Note: 3rd party lead vendor should know the Lead ID that they will delete.
From the Update Lead tab. Scroll down up to Request Query String.
Put a check mark on Show Query.
They will also received a JSON Response on the lead that they deleted.