My discourse is invite-only to members of a paid community I run.
For those who, for example, fail on a monthly payment and don’t resolve it…
I’d rather this was temporary, not permanent, in case they resolved the membership payment, or decided to join again further down the line.
Thanks 
There is no direct way to achieve this but you can easily work around this by adding/removing such users to a group and tweaking category permissions so that only this group can participate in the discussions.
Ah, OK. So like I could put them in a group which literally gave them no access/permissions, right?
Would they still be able to log in? Ideally, I don’t want them to be able to see or participate in any threads/categories. (i.e. have completely zero access)
If you want to deny them from logging in, The best way would be to use a 3rd party access management system and deny their access using your IAM system instead of discourse.
The easiest way to accomplish this is suspending the user’s account until they pay for the current month.