<event name="payment_method_is_active">
<observer name="custom_payment" instance="OpsKgPayMethodObserverPaymentMethodAvailable" />
</event>

<?php

namespace OpsKgPayMethodObserver;

use MagentoFrameworkEventObserverInterface;

class PaymentMethodAvailable implements ObserverInterface
{
    public function execute(MagentoFrameworkEventObserver $observer)
    {
        if( $observer->getEvent()->getMethodInstance()->getCode() == "banktransfer" )
        {
            $checkResult = $observer->getEvent()->getResult();
            $checkResult->setData('is_available', false);
        }
    }
}