Stupid question about a potential attack(?) against steemit !

If competitors use Steem Power to attack us...

What would happen if a competitor created some accounts and fairly Powered them Up to...

1.Downvote Good Content and
2.Upvote bad content?

Would an attack like this ruin our platform?
How could we defend in such a situation?

PS Same question in a lower scale....
Let's suppose a user with significant Steem Power don't like us, and starts to downvote each post of us. what can we do about it? Stop using the platform?