You can't delete their Codio accounts (Students can do that themselves if they do not wish to carry on using Codio in the future) but you can remove them from your organisation so the user licenses released can be re used.

See Delete Course  
​You can delete just the course or you can also delete the course AND remove the students from your organisation as well. Be aware that if you select the option to Delete the Course and Students and any of the students in this course are also members in other courses, they will not show in the list of students and will not be removed from the organisation or other courses they are also members of. The option Delete Course Only will just delete the course but not remove any students from the organisation
Alternatively, you can remove them individually from the organisation. See Remove Members