If you do not need to use WPSOLR anymore, you can delete your subscription. This will also cancel your (eventual) billing auto-renewal.
Open your subscription (aka license) Login to your account
Delete the subscription
What happens next?
Your subscription license is no more valid, and your current active plugin can no more receive updates. You should deactivate and remove the plugin also on your website.
Also, your 2Checkout subscription was cancelled, and its auto-renewal deactivated. Therefore, no risk to be debited on the next renewal. You can log in your 2Checkout account, and verify your subscription status if necessary.