Tag areas that are set to "forced" standard tag usage are supposed to prevent tags from being added on the fly and only allow tags to be selected from the tags drop down.

Steps to recreate:

1. Go to the Manage Tags page <URL>/tag/manage.php
2. Add a tag collection
3. Associate the "Questions" tag area to the tag collection
4. Set the Questions tag area to "force" in the Standard Tag Usage column.
5. Confirm that there are question tags existing.  If not add a few.
6. Edit a quiz
7. Edit a question
8. Go to the Tags section and manually key in a tag name and press Enter.
9. You should NOT be able to add the tag because the tag area is set to "force", but it allows the tag to be added.

Last Moodle version where this is confirmed to be working:  Moodle 3.4.1