Yes, you can always deactivate the Windcave Payment Processor Integration plugin from the Purchase Subscription screen.
You can approach this screen either for the Plugins screen or from the Manage Account section.
If you are coming from the Plugins screen, you need to click on the Deactivate button on the plugin tile labelled Windcave Payment Processor Integration.
If you are coming from the Manage Account section, you need to navigate to the Already Active section, on the Purchase Subscription screen, and remove the tick from the activation checkbox to mark the plugin as Inactive.
Upon deactivation, all the features and functionality added to the application are removed. However, you can always choose to reactivate the Plugin without incurring any additional charges, during its subscription cycle.