UPDATE: Elastic Email can now work with Discourse admins to disable the EE unsubscribe link and manage subscriptions through Discourse.
I raised the issue with their support and received this reply:
Hi Dan,
Some of our customers are using discourse.org and we are quite happy with their sending statistics. In other words, they are all legitimate businesses which we love to service.
Considering your circumstances, we will work with you and disable unsubscribe link. This way you will be able to manage subscription through Discourse.org.
I hope that it works for you.
Thanks
Lubomir