Indeed. Things like "Ignore User" are only useful when it's one half-hearted troll. When you get a dedicated troll or, worse, a mob of trolls, it's a stop-gap measure you can use to plug holes in the dam as the wall falls down around you.
A better option would be to allow users to auto-ignore accounts that are under a specified age (perhaps with an option to exempt "verified" accounts). This doesn't completely preclude multi-account attacks--either through hacking existing accounts, buying botnet accounts, and/or creating a cache of accounts for future use--but will make it extremely prohibitive for trolls to attack in the heat of the moment, as they did Zelda Williams.
To deal with the zealous troll, then, you would have a final defense: an account lockdown where the only messages you see/receive are from users you've personally friended/followed. (So none of that bullshit like Pintrest making you a follower of a few thousand accounts; why is that even a fucking thing?) It would be nice to have a complimentary system where you can grant other accounts (either people you trust or, for larger celebs/businesses, paid staff) the ability to whitelist individual messages so you aren't completely cut off from fans/followers in general (and the morale boost they might bring.)