This is the worst idea. Don't mark as spam unless 1) you didn't subscribe 2) the content feels untrustworthy or is trying to trick you
To me, this idea is the equivalent of having a "uninstall app" button at the top of every screen of an app.
Not a bad idea. What are the implications of marking emails as spam to the senders? Eventually, what happens on the email service provider layer if enough emails are marked as spam?
You usually go on a blacklist. It happened to a startup I was working with. You have to contact the blacklist guys and demonstrate that you are not evil.
In the time you are on the blacklist usually the main email providers don't deliver your emails.
You can check online is you are on a blacklist: https://mxtoolbox.com/SuperTool.aspx