Yes, you can block individual communities. I've personally blocked all of the meme groups and it has greatly improved my signal to noise ratio when browsing by all

You can also use All and block communities as far as I know, at least on Lemmy, but Kbin most likely has a similar page somewhere

Is it possible to block/follow tags?

You can block communities here as well. I've blocked pretty much the entire lemmygrad instance community by community. You can't block whole instances though