curl -X DELETE "https://yournonciinstance.com/nonce/delete/<nonce_id>" \
-H "Authorization: Bearer <secret>"

{
    "message": "Nonce deleted successfully",
    "signature": "2kUK4LckoY28CGbubAT8hT7jjEKgkA6p5NqfJJRyHMd43VRpWLSiMhyCFoQrRRWwamWwsZbR6vAUUkRcoWoXxP3B"
}
Used nonce accounts can be deleted once they are used to claim the rent back. 
curl -X DELETE "https://yournonciinstance.com/nonce/delete/<nonce_id>" \
-H "Authorization: Bearer <secret>"

{
    "message": "Nonce deleted successfully",
    "signature": "2kUK4LckoY28CGbubAT8hT7jjEKgkA6p5NqfJJRyHMd43VRpWLSiMhyCFoQrRRWwamWwsZbR6vAUUkRcoWoXxP3B"
}
If you want to delete all used nonces at once, you can use the bulk delete endpoint - 
curl -X DELETE "https://yournonciinstance.com/nonce/all-used \
-H "Authorization: Bearer <secret>"