First of all I think I should state that I'm probably missing something pretty obvious.
What I need to do is make the site "Members" unable to Add Column on a list. I planned on changing the permission level from Edit to Contribute at the main Members Group Level but I was unable to do that, the option to "Edit User Permissions" under Modify when I select the group wasn't available.
I then broke the inheritance on a list and changed the permission level to Contribute like so:
But my member user can still Add Columns on the list. Am I just being daft here?
*Edit to add. Switching to Classic mode this works as expected. So I guess my question is how do I disable the Add Column button in Modern Lists?