How can I get rid of unwanted "this computer only" Calendar and Contacts?

This is an IMAP account? The folders in the IMAP data file are marked this computer only because they don't sync and are available only on this computer. You can set a pst file as default in Account Settings, Data files then delete the imap ost and resync. The pst folders should be used for calendars and contacts.

I have the same issue. I have just installed Office 365 and am using the version of Outlook that comes with that product, which I think is Outlook 2013. I have a number of email accounts. I used the automatic process to set up each account, and they were all created as IMAP accounts, and it doesn't seem possible to change them to POP accounts. An Outlook Data file has been created for each email account, but they are all .ost files. You say "You can set a pst file as default in Account Settings", but in account settings I can only see this list of .ost files. How would I create a pst file which I could then set as default?

Assuming 'general contact folder' is in a pst, select all, copy and paste or using the Move to folder command. For the calendar, use a list view and then select all, copy/paste or move to folder. 

If you don't have a pst in the profile, add or create one, set it as default data file then move the contacts, calendar, tasks, and notes if used. After everything is moved, close outlook and delete the imap data file to get rid of those 'this computer only' folders. 

I have a tutorial at https://www.slipstick.com/outlook/2013/imap-accounts-outlook-2013/

I created the pst file, set it as default. This solved my Calendar and Contacts issues, but mail still kept going into the IMAP folders for the various accounts. Eventually I deleted them all and re-created them as POP3 accounts, and I can now direct the incoming mail to an appropriate .pst folder. The only problem was that during the time I had my main email account set up as an IMAP account, the entire content of my Inbox was loaded back up to the server. So when I re-instated the account, it downloaded everything again and I had two copies of about 3,000 emails. The trap for me was the automatic creation of IMAP accounts, rather than POP3 accounts by Outlook. I think there needs to be a warning message for those who haven't encountered IMAP before that this may not be the way they want to go.

Go to File, Account Settings. Switch to the Data Files tab. Click New... and create a new pst file.  Set it as default. close and reopen outlook. Check all imap folders for calendar, contacts, tasks, notes. move them to the pst folders. Close Outlook. Delete all of the imap psts and restart Outlook. The mail will download but the special folders won't be created.

See IMAP accounts in Outlook 2013 for more information and a tutorial.