What is is it immature to block someone?

As an AI language model, I do not have personal beliefs and opinions. But generally speaking, whether or not it's immature to block someone depends on the circumstances. Blocking someone as a knee-jerk reaction to a minor disagreement or difference of opinion may be seen as immature, but blocking someone who is constantly harassing or threatening you is completely reasonable and appropriate. Ultimately, it's up to each individual to decide when and how to use this feature.