Hi @warandpeass,
That is no fun, but fortunately this issue has been raised before. Please download, install and activate the following plugin:
https://www.dropbox.com/s/op9j1wt9kx6gwiq/wp-custom-admin-interface-delete-settings.zip?dl=0
This plugin will delete all WP Customer Admin Interface settings (so you will need to setup things again, but at least you can continue to use the plugin on your site!).
Once the plugin is activated it will delete all settings, there’s no configuration required. Just remember to delete this plugin once you are done otherwise the plugin will continually keep trying to delete the settings so nothing new will save.
Whilst have the ability to write custom PHP is handy, I don’t recommend actually using this feature and I tempted to just remove it from the plugin. I recommend making PHP edits via FTP, that way if an issue arises you have the ability to immediately undo things.
Thanks and best of luck ??