Disable Payment Method for Admin in Magento 1

We should take a look at the payment abstract class vendor/magento/module-payment/Model/Method/AbstractMethod.php. We can see the method public function isAvailable() - This class will check whether payment method can be used.

   public function isAvailable(\Magento\Quote\Api\Data\CartInterface $quote = null)
    {
        if (!$this->isActive($quote ? $quote->getStoreId() : null)) {
            return false;
        }

        $checkResult = new DataObject();
        $checkResult->setData('is_available', true);

        // for future use in observers
        $this->_eventManager->dispatch(
            'payment_method_is_active',
            [
                'result' => $checkResult,
                'method_instance' => $this,
                'quote' => $quote
            ]
        );

        return $checkResult->getData('is_available');
    }

As can we see, there is an Observer event, like Magento 1, we can use Observer to disable our Payment Methods. It's Magento 1 way. However, Magento 2 introduces to developers a new way - Plugin.

Create DI on front page: app/code/{Vendor}/{Module Name}/etc/frontend/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Payment\Model\Method\AbstractMethod">
        <plugin sortOrder="5" name="testValidate"
                type="{Vendor}\{Module Name}\Plugin\Model\Method\Available" sortOrder="1" disabled="false" />
    </type>
</config>

app/code/{Vendor}/{Module Name}/Plugin/Model/Method/Available.php

<?php

namespace {Vender}\{Module Name}\Plugin\Model\Method;

class Available
{
   /**
     *
     * @param \Magento\Payment\Model\Method\AbstractMethod $subject
     * @param $result
     * @return bool
     * @throws \Magento\Framework\Exception\LocalizedException
     */
    public function afterIsAvailable(\Magento\Payment\Model\Method\AbstractMethod $subject, $result)
    {

        if($subject->getCode() == 'your payment code, eg: cashondelivery') {
            return false;
        }
        return $result;
    }
}

$subject is the current Payment Method Model.

Best idea to use event/observer concept.Fire an observer on event: payment_method_is_active

Create events.xml at app\code\[Vendorname]\[Modulename]\etc\

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="payment_method_is_active">
        <observer name="disable_pg" instance="[Vendorname]\[Modulename]\Observer\ObserverforDisabledFrontendPg" />
    </event>
</config>

then at observer class ObserverforDisabledFrontendPg.php rewrite below code:

<?php
namespace [Vendorname]\[Modulename]\Observer;

use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\App\Request\DataPersistorInterface;
use Magento\Framework\App\ObjectManager;

class ObserverforDisabledFrontendPg implements ObserverInterface
{
    protected $_appState;
    public function __construct(
        \Magento\Framework\App\State $appState
    ) {
        $this->_appState = $appState;
    }

    /**
     *
     * @param \Magento\Framework\Event\Observer $observer
     * @return void
     */
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
      $result = $observer->getEvent()->getResult();
      $method_instance = $observer->getEvent()->getMethodInstance();
      $quote = $observer->getEvent()->getQuote();

      if(null !== $quote){
          /* 
          *  During Checkout magento call payment methods from both
          *   area frontend and Web_api are
          */

          if($method_instance->getCode() =='checkmo' && 
            in_array($this->_appState->getAreaCode(), $this->getDisableAreas()))
            {
               $result->setData('is_available',false);
               //$items= $quote->getAllVisibleItems();    
          }
      }

    }      

    protected function getDisableAreas(){

        return array( \Magento\Framework\App\Area::AREA_FRONTEND, \Magento\Framework\App\Area::AREA_WEBAPI_REST);
    }


}