Click the "flag" link under the submission title. Edit: apologies, I just read on the guidelines that: "If you think something is spam or offtopic, flag it by

Click the "flag" link under the submission title. Edit: apologies, I just read on the guidelines that: "If you think something is spam or offtopic, flag it by going to its page and clicking on the "flag" link. (Not all users will see this; there is a karma threshold.)" I just upvoted to help you on your way :)