The following API call allows voiding a bill-payment in QuickBooks Online: 

curl -X PATCH \
https://api.openconnectors.us2.ext.hana.ondemand.com/elements/api-v2/bill-payments/IdOfTheBillPayment/void \
-H 'Authorization: User *****, Organization *****, Connector *****' \
-H 'accept: application/json' \

After the bill-payment is voided, a privateNote with the Voided message appears in the response body of the bill-payment. Here is an example of the above API response:

{
 "checkPayment": {
 "printStatus": "NOT_SET",
 "bankAccountRef": {
 "name": "10100 Checking",
 "value": "58"
 }
 },
 "systemId": "8730",
 "syncToken": "1",
 "vendorRef": {
 "name": "Patton Hardware Supplies",
 "value": "11"
 },
 "metaData": {
 "createTime": "2019-06-17T05:54:39Z",
 "lastUpdatedTime": "2019-06-17T06:19:30Z"
 },
 "payType": "CHECK",
 "totalAmt": 0,
 "privateNote": "Voided - RC 93",
 "sparse": false,
 "currencyRef": {
 "name": "United States Dollar",
 "value": "USD"
 },
 "exchangeRate": 1,
 "domain": "QBO",
 "id": "8730|1",
 "txnDate": "2019-06-16T00:00:00Z"
}