In the course form, select the resource, right-click and choose Remove the resource from the course, or use the key [Del].
To remove a resource from a course, you can also hover over the concerned resource and click on the x-cross to delete it.
Select the data and press on the key [Del] or right-click and choose Delete (also works in multi-selection). Only the data that is not used elsewhere can be deleted; if needed, delete the absences, courses, etc. of a resource before deleting it.