Outlook 2011 - Cannot cancel large synch operation, cannot remove and start anew
Have you tried to rebuild the database and/or setup a new identity to 'try again'?

The first, rebuilding the database, is quite easy:


Make sure Outlook is not running
Hold the option (alt) key
Open Outlook 2011
Select your default profile, presumably Main Identity and select the [Rebuild] option
Wait... This could take a while.


The second option, creating a new identity is done in nearly the same way as the previous steps. Instead of selecting [Rebuild], select the plus sign [+]. This will allow you to create a new, second identity. Once configured, it'll get the state as known and persisted by the Exchange server itself. 

As a second note: Normally, even with very large quantities of 'changes' in an Exchange account the sync should still be pretty reliable. Before trying again, have you looked into updating all latest servicepacks and updates through Help > Check for updates? There are lots of changes in the last month and several patches to improve sync as well as stability have been released by Microsoft to prevent problems you're experiencing.