What are you trying to delete the task itself or the trigger?
If you click on the application in the you should see a red X to delete the task.
If you are in the triggers tab a red x should be next to the trigger.
If you are not seeing this please refresh your console (F5). Also on the status page do you have Automatic refresh of task list checked? If not you will have to always manually refresh to see results.
I'm having the same problem with SR4. It appears that the Source Documents page in QMC is not scaling properly. I see the task description next to the document, but no red X, no pencil, and no run button. However, if I zoom out (ctrl -) until the page is almost too small to read, I can see those icons! I was able to delete by clicking on the tiny zoomed-out icon.
I thought this was just my browser, but perhaps a bug?