Check the filter module documentation: http://docs.question2answer.org/plugins/modules-filter
You just need to write a plugin with a filter module that implements the filter_question() function and block the creation of the question if there is any non-existent tag.
Taking it a step further you could check the level of a user (admin, registered user, etc) and decide if that user can (or cannot) create tags.
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April 20, 2021: Q2A 1.8.6 released!