code: public CoinbaseBaseResponse cancelCoinbaseRequest(String transactionId) throws IOException {
  
      final CoinbaseBaseResponse response =
          coinbase.cancelRequest(
              transactionId,
              exchange.getExchangeSpecification().getApiKey(),
              signatureCreator,
              exchange.getNonceFactory());
      return handleResponse(response);
    }
docs: Authenticated resource which lets a user cancel a money request. Money requests can be canceled
  by the sender or the recipient.
  
  @param transactionId
  @return true if canceling the request was successful.
  @throws IOException
  @see <a
  href="https://coinbase.com/api/doc/1.0/transactions/cancel_request.html">coinbase.com/api/doc/1.0/transactions/cancel_request.html</a>