A word of warning when deleting a community: there will be threads and people who have posted questions in there. You need to make sure that you're not disrupting your course, site, or membership community. For the sake of providing deletion information, an email broadcast should be sent out to students, informing them of any deletions or changes to the courses or communities they are part of.
To delete a community, go to 'Communities' > select the desired 'Community' > click the 'Delete' button.
This window will open up.
If you click 'OK', that community will be deleted now, as you can see below it is not listed below anymore.
But if you wish to cancel this, simply hit the 'Cancel' button.
Also, watch the video on 'Deleting a Community'