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Yes, votes from new members with few Steem Power, are not so many worth. But these new members will grow and upvotes from them will become higher in future.

does a post pay after 7 days if still getting upvotes? Is there any way to downvote?

No, when you got your payout after 7 days, you got nothing more when a post got upvotes after the payout.
You can downvote on the steemit page with the flag in the top right area of a post or comment. At steemfollower you can add negative values between -0.01 and -100 for upvotes.
But you should use flags/downvotes only in very rare cases when a user does not behave properly, e.g. (plagiarism, illegal content, insults).
But it is better if you point out the user's error within a comment, instead of flag him.
If you don't like what a user wrote, you can also mute this user.

if I give -100 flagging someone.. does it also hurt me?

Also when you give a flag, your voting power goes down. So you can give one upvote less to others. Also if you flag a big player with a lot of steem power, these user could take revenge and downvote you down to level 0.

level 0?

New users start with reputation level of 25. But you can go down to 0 (or maybe 1) if you got many downvotes (or one really big downvote).