In 2005, if you go to the Site Settings page and you have full admin rights on the server you should see a link for "Manage Jobs." If you then go to that page and refresh a couple of times you should be able to see the job you want to cancel.

Check the box, hit the cancel and it "should" kill the report.

I say should because I've had times where it didn't and I had to look at the start time in running jobs and match that with a SPID and kill the SPID.