It's complicated. Basically Mastodon can see anything that's public and there's no easy way to prevent this, because the information is public. When the plugin is disabled, the channel discovery information should no longer provided for that network, but if they've already cached the information they don't need to fetch it - as they already have it stored locally. And there's also a bug. I'll do what I can and fix the bug, and see if there's a way to block that network after you've allowed it at some point in the past; but it is still complicated.