How to block specific keywords from searching on WordPress?

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You are looking for pre_get_posts. This action is run between the composition of a (search) query and the actual query, to allow you to change the query. So, it would allow you to intercept banned keywords like this:

add_action( 'pre_get_posts', 'wpse338558_intercept_banned_keywords' );
function wpse338558_intercept_banned_keywords ($query) {
  $banned = array ('word1','word2','word3');
  if (in_array ($query->query_vars['s'], $banned)) $query->s="";
  }

Notes:

  1. The above will simply empty the search, so there will be no feedback to the user. A hackish solution would be to use a very obscure search term like jlwenuhrofslslfiehlserlisehrlilfheeipeueiq and intercept that 404 to display a message.
  2. The above will only intercept single keywords. If someone searches for two banned keywords at the same time he will get through. So you’ll need a more elaborate condition to make this foolproof.

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