It's a feature, not a security flaw. Blockchain based platform is censorship resistant so you can't delete anything that other users have written.

In the future there will be other user interfaces to the blockchain (besides Steemit). Probably some of those will have a feature that gives rights to users to hide comments that they don't like.

Edit: Actually there is one way how this can be implemented without breaking anything or taking away essential features from the blockchain: https://steemit.com/steem-ideas/@samupaha/feature-proposal-ownable-tags