Flagging or downvoting lowers the user's reputation and the post-payout if it has one. It can also result in the post not being "seen". Posting claiming to be someone you are not is frowned upon by the community, even in jest if there isn't a disclaimer saying "This is for fun, I'm not the President." or "this is satire".
You can find the flag icon in the top right of any post or comment. I recommend only using it in the case of abuse. You could get revenge flagged.
Oh now I see the flag button. I had previously not.