Simply put a list of Disallow entries with your category names in the robots file (beware that categories are also accessible under the path "/questions"):
Note that if your robots file contains a sitemap URL (which it should) you need to remove the category entries from the sitemap as well. However, the Q2A default sitemap plugin only allows for omitting the category browser, not the individual category entries (https://example.org/questions/category-A, https://example.org/questions/category-B, ...). Hence you need to either remove the sitemap (not recommended), or use a different plugin for generating it without category entries.
There is at least one more advanced sitemap plugin available, but I don't know if it supports omitting categories, since I haven't used it myself. You may need to patch the code either way.