OK, so red-thumbing probably does work as intended, but this person is flouting that. Continue to red thumb them. If you can prove it is the same account, report them (if there is an ongoing campaign of sabotage, you should have tables to reference). I'm curious how you expect the red thumb system to work with someone who wants to flout the system? If they're willing to do that, they're probably willing to do other things to get around whatever additional protections are put in place.
In the meantime, you could also try to find some like-minded Spades players and friend them or add them to a group. Play only with that group. Maybe occasionally venture out of the group to see if you can find some new players to add. If you admin the group, you can kick out the problem player if they manage to make their way in.
I don't disagree that there should be better privacy settings available, but I'm not sure how being able to hide your online status (or the other suggestions mentioned there) would help if you've already red-thumbed this person and they're still joining your games.
