If your organization is no longer using a Release (perhaps because it's finished), you may mark that Release as "inactive." Doing so prevents the Release from automatically appearing on the Release screen or in any drop-down menus.
Note that if a Release has no Deliverables, you can't mark it inactive, but can delete it instead.
Go to the Releases screen and click on the name of the Release you want to mark inactive.
Select the "Tools" tab.
Click the "Mark Inactive" button.