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This question already has an answer here:

How do I disable the activities overview screen from popping up when I hit the top-left corner of my screen in Ubuntu GNOME 17.04?

marked as duplicate by karel, Charles Green, Zanna, Eliah Kagan, storm Oct 8 '17 at 13:19

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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There are many extensions which achieve this purpose, for example:

  1. No Topleft Hot Corner
  2. Activities Configurator