Customers are unable to delete completed sprints in team-managed projects.
A completed sprint will not appear in the team-managed project's Backlog or board. But customer still can see them in reports.
If they want to delete completed sprints from reports, The only option is to reopen the sprint first, and then delete it from backlog. There is no direct way to do this.

Previous Feature request without solving the "delete" option.

Steps to reproduce:

1. Go to the project where your board is located, then select your board from the Board menu.
2. Click Reports, then select Sprint Report.
3. Select the relevant sprint from the sprint drop-down.
4. Select ... > Delete sprint does not exist. Only "reopen" option is there.

Workaround:

1- Go to Reports > Sprint Burndown Charts
2- Choose the sprint you want to delete from the drop-down menu.
3- Click on ... and choose "reopen sprint:

4- Go to backlog again and delete the sprint from there.