The "New contributor" indicator that ask us to welcome the user showing on a spam post.
Perhaps that is the problem. The "New contributor" indicator does not "ask us to welcome" them. Neither the indicator itself nor the tooltips on them tell us to welcome them. The most we are asked to do is "be nice". Which ought to go without saying, but that's another matter.
My point is that you're misinterpreting the meaning of the indicator. It merely is there to remind us that the person is a new user.
This is ultimately no different from seeing the "New contributor" indicator on a terrible post. How do you treat a terrible post? You downvote/VTC as appropriate. How do you treat a spam post? You flag it as spam.
The indicator is just there to remind you that the poster is new. Do not allow it to change how you interact with the user's content.
From the post showing off the indicator:
The new indicator doesn't alter functionality.
It's a reminder that how you interact with the person could be extra meaningful (any time we help strangers just out of the love for what we do, it's meaningful). But it's not a magic shield of any kind, it doesn't work like spawn protection (where players can't take damage for the first few seconds after entering) in video games.