Well, I guess we should consider it as one of the trials of freedom. Either you live in a walled garden, or you have to be conscious about what you're interacting with and exercise critical thinking. If you don't want to be associated with genocide supporters - don't register on tankie instances, or even don't subscribe to any content from them. It should be a personal decision of every user (or instance maintainer for that matter - defederation is also an option, simply because instance maintainers are also people).
Another important thing is to speak up about it - to raise awareness, so to speak. It's perfectly reasonable to state what the worldview of tankie admins is - in threads like this, as well as on other platforms. I was considering lemmy.ml as Reddit replacement at first, but saw a comment on one of the Fediverse sites and digged more into this.