Go to...
Outlook 2010/2013
- Open the calendar screen in outlook (refer to the image below, label 1)
- Find the calendar name you wish to delete in the navigation pane (refer to the image below, label 2)
- Right-click the calendar name and select delete calendar from the drop-down menu

IOS 7/8/9
- Navigate to the settings app
- Locate and open the "Mail, Contacts, Calendars" settings (refer to pane 1 of the image below)
- Tap/Select the subscribed calendar which should appear under the "Accounts" section (refer to pane 2 of the image below)
- At the bottom, press the delete button which will remove the calendar from your device (refer to pane 3 of the image below)

Android 4.4+
Note: These instructions only refer to the default "Google Calendar" app
- Open the Menu, then Settings
- Tap on your email account
- Uncheck the calendar you want to unsync