I no longer have access to the account from which email forwarding is set up. But the emails continue to come to my original account and I could find no way to disable it.
You can disable email forwarding only from the account which forwards. As you don't have access there you obviously cannot do that.
What you can do is to add a filtering rule on your target account which would delete forwarded emails.
Additionally having all emails forwarded may allow you to reset password and regain access on the forwarding account, have you considered that?
As Tomek said, you can't make changes to an account you don't have access to, so stopping the forwarding yourself is likely not possible. (You could perhaps make a support request to the operator of the service, asking them to stop the forwarding. They might do that, with sufficient proof that you're the owner of the recipient account.)
But assuming you want to do this yourself, the best approach would indeed be to configure your gmail account to automatically discard those messages. The first step would be to find a way that you can match all of those messages, and only those messages, in order to filter them.
Gmail has multiple search keywords you can use in search/filtering, and one handy one for dealing with forwarded messages is
deliveredto. For most types of forwarding, searching for
deliveredto:firstname.lastname@example.org will find any messages forwarded from that email address to yours.
If you perform that search and it does match all of the correct messages, then you can choose "Create Filter", and select an option like "Delete it" or "Skip the Inbox (Archive it)" (along with "Never mark it as important" and "Mark as read" just to be sure it stays out of your unread/important messages) to have Gmail handle the forwarded messages automatically in the future. (And you can also apply it to the existing forwards, if you choose.)
I know this is an old question but there is a way to disable email forwarding from the receving address completely provided that the account provider is gmail.
Look for a mail by email@example.com with the subject containing
Gmail Forwarding Confirmation. This is the mail you recieved when you setup email forwarding in your accounts. Scroll down the text body of this email and you would find a link to cancel email forwarding. In my case the text body had this:
If you accidentally clicked the link, but you do not want to allow firstname.lastname@example.org to automatically forward messages to your address, click this link to cancel this verification:
followed by a link. Clicking the link redirects you to page which confirms that you have cancelled the email forwarding and will no longer recieve emails.
However, I do not know whether the link expires after some time or not. In my case, it was still active after 3 weeks