You go to their comments, copy the link, then open up Steemit.Chat, go to Steemcleaners Linkdrop, and leave your report with the link in there.
You can also report them in the comments for the @steemcleaners posts, but if they're getting notifications (such as if they're using eSteem, etc) they'll get a notification for being mentioned. They won't if you just use the linkdrop in Steemit.Chat.
Thank you.