If you don’t want to see spoilers, it’s your responsibility. Other people can put whatever they want on their blogs.
If they haven’t specifically stated that their blog is going to be spoiler-free, assume that they will be posting spoilers. Use Tumblr Saviour. Temporarily unfollow. Maybe, I don’t know, stay off the internet.
Nobody is forcing you to, but equally nobody should have to restrict what they put on their blogs, purely so you can use Tumblr without seeing spoilers. If you don’t want to see them, it’s very easy to avoid doing so.