try these, one of em should work:

Open the message from the Outbox and close it again. Now select the message
and press DELETE
                                         or
Put Outlook in offline mode. To do this go to File-> Work Offline. Now
restart Outlook and see if you can delete it (you can try this in combination
with method 1). Don't forget to put Outlook back On-line again.
                                         or
Create a new pst-file; File-> New-> Outlook Data File...
Set this pst-file as your default delivery location in your Account
settings; Tools-> E-mail Accounts-> button Next
When you now restart Outlook your original pst-file will show up as the
secondary set of folders where should easily be able to remove the file from
or even the complete Outbox folder
Set the original pst-file as the default delivery location again and restart
Outlook; the Outbox folder will be recreated and will be empty.