It's very odd that the option is not available. As below, to register email addresses as spam, you need to select the email and tap MORE, then Register as spam.
Can you provide a screenshot of what you're seeing? Do the options appear different when selecting different emails?
I have the same issue except that I can mark it as spam but since spammers never use the same email I get inundated with spam every day. This never shows up on my computer, just the phone. Phone carrier is not the issue, it is this app that has a very weak spam filter and no way to address it. We need a real filter, not just going in and marking every one that comes in!