Deleting an Idea is easy. Navigate to the Ideas page, and find the Idea you want to delete. Click on it, clicking on the Idea will cause it to expand, and the settings button will be visible:
Now, simply click on delete:
This will prompt a window asking whether or not you are sure you wish to delete the Idea. If you are sure, click "OK", but if you have changed your mind click "Cancel". Ideas cannot be recovered once deleted.