How can I restrict access to my Discourse forum for users who have stopped paying for my membership?

[simon]

Ideally when someone cancels membership with my site, they should also lose access to my forum. When they try to access the forum they should be sent to a specific page on my website where they can update their billing information in order to re-gain access to the forum.

I am assuming you are using the WP Discourse plugin. There is nothing built into the plugin to handle this, but it can be done by adding some code to your site. Have a look at this post: How to prevent some WP users from being able to login to Discourse.