Create canned response with text:"Multiple issues per ticket is against policy. Staff are reminded to submit each issue individually."Close each ticket with multiple issues replying with that response.. eventually, after you've closed a dozen tickets or so for each of them, they will learn. Or, because you will have to copy the text relevant, and any attachments, its possibly easier for you to simply hit the "New ticket" button and add the relevant details, you can even assign from there. They will get a reply for EACH issue, and may pick it up that way.