canceling subscribers to my blog

Awhile ago, I added a blogtrottr signup form on my blog. A few people have signed up for emails whenever I update the blog.

Recently I switched to a different service and re-signed up the people who had previosuly signed up with the blogtrottr form. Now they get two emails everytime I update my blog (one from my new service and one from blogtrottr).

How can I end the blogtrottr email blasts? I know I can tell them to unsubscribe, but that seems a little sloppy and there is the risk that they will unsubscribe from the wrong service.

Thanks for any help.