I don't like an activity

At the bottom of the activity, click the button that says "Report" and explains clearly the reason why you don't like this post and why it should be deleted or edited.

If the same user posts a lot of activities that should be deleted, go to their profile and under their avatar, click the "Report" button.

DO NOT click the "Report" button on accounts if there is a problem with their activities, since the only thing it will do is remove the account from the leaderboards, they will still be able to post activities.