That only works if the abuser is the only subber. Otherwise, innocent subbers’ subs will get lost, as well. CMs cannot selectively delete the subs of one person in particular. Only Viki Staff can do that and they usually don’t. Even if they delete the abuser’s account, the subs often stay.
Are you saying that NF is lacking subtitles in many languages? That’s actually not the case. For most shows, there are several options, but the options shown to you are based on your settings. In some cases, the options are indeed limited, but that may have to do with license issues or even with a lack of translators. And even Viki has such limits.