Need to be able to mark specific emails as spam, even if there is an existing reply taking place.

Right now, if someone forwards me an email to verify if it's spam or not, I can't mark the email as spam. It gives me a warning and says that I would be marking my company as spam.

In other words, if there is an ongoing email thread taking place (sometimes people reply to an email, then realize it's bad), I'd like to be able to choose the email that is spam and mark it specifically so we don't keep getting their emails. Otherwise I have to find a workaround to block them.