Never tell a reporter that ‘it is off the record’. You can ask them to put something off the record but you cannot tell them to do so.
You may see it happen in movies and TV shows but that is not how it happens in the real world. A reporter does not have to grant you permission to go off the record. Some media organizations do not even let their reporters go off the record with sources. So you need to ask first if you want to go off the record. You need to see them nod their head and say “Yes” as a reply. Only then is when you go off the record.
The reporters are not your employees and you are not their boss. You need to ask them first. Secondly, you need to have verbal and possibly visual assent to your request to ensure that you are, indeed, off the record.