Found the solution myself. Turns out there is a "Set as default notebook" option in the menu, and the notebook that is currently set as the default is disabled from being deleted. So, select another notebook from your account and set it as the default. Now you are able to delete the previous notebook.
Some feedback for Evernote: if only there was some indication (either in the menu options, or somewhere in the Notebook list view, or anywhere else for that matter) which indicated that "This notebook is set as the default notebook" it would have really helped. Otherwise, the only way I coul