Thanks for the comments and suggestions.
In this case, we are indeed managing logins externally and are disallowing access to the forum based on WP membership IDs with the wpdc_sso_provider_before_sso_redirect hook. But that sounds like a maybe moot point and the suspension method will work for us.
The other question I had, which I answered with a bit of search, was regarding email summaries and notifications for suspended users. It sounds like they might still receive some PMs, but otherwise no emails will go to them if suspended.
I assume this would also be similar for the custom group method, though perhaps a bit less elegant.. ie. a member is expired and removed from the main ‘member’ group but could still theoretically receive some form of activity summaries if there are categories visible outside the ‘member’ group.
Thanks
Ryan