Quick article from the Microsoft Office 365 Page on how to empty your deleted items folder.
Empty the Deleted Items folder
Outlook can be configured to automatically empty the Deleted Items folder, or you can manually empty the folder at any time.
Note: If you're using an Office 365, Outlook.com, or Exchange account, you'll have a Deleted Items folder. If you're using a different type of account (like Gmail or Yahoo or another email account), your folder will be named Trash instead.
Manually empty the Deleted Items folder
In the folder list, right-click the Deleted Items folder or the Trash folder, and then click Empty Folder.
Automatically empty the Deleted Items folder
- Click the File > Options.
- Click Advanced, then under Outlook Start and Exit, select the Empty the Deleted Items folder upon exiting check box.
To be notified before the Deleted Items folder is emptied automatically, scroll down on the Advanced options screen to the Other section, and select the Prompt for confirmation before permanently deleting items check box.