It might be better to cripple the site rather than kill it. If all of Mahalo's pages disappeared you can be sure they would return en masse when they found the workaround for the filter. Blocking chunks of their content might make finding the workaround harder and may ultimately force them (or any other low quality site) to up the quality - yeah, I know I am deluding myself.
If they're serious about their standards, they would remove Mahalo en masse from their index.
edit: Or, to satisfy lukev, they can keep the index as-is but make sure Mahalo pages never rank high in results.