The release of Kingdom Hearts 3 has hammered home an increasing problem on Youtube: spoilers. Youtube's algorithm's are based on engagement. Negative engagement is still engagement. Getting upset about a video spoiling something for you keeps you emotionally invested in their platform. Naturally, there are no proper built-in blocking tools for Youtube. Why would Google want you to control your experience?
Thankfully, there are browser extensions that will let you prevent channels and videos from appearing on Youtube without your consent. By using these extensions, you will no longer see videos from your blocked channels on Youtube's front page, search results, or recommendations. If you click directly through to a channel or video from a link on an external site, however, they will not be blocked, so be careful.