|Jira Agile 6.3
If you have Sprints created just for test and/or have created sprints by mistake you cannot simply remove, however you are able to clean the Sprint field value from all issues of the desired Sprint.
Run a query on JIRA Issue navigator searching for: Sprint = <sprint number>
Bulk Edit the field Sprint for all issues on the search;
Remove the sprint number and complete the Bulk edit;
Make sure you delete the right Sprint number, otherwise you will remove the issue from other Sprints
Check the Rapid Board and note that there is no Reports related to Sprint 5.