Is it possible to set the search field so it only searches knowledge base articles and offers no other options?

Is it possible to set the search field so it only searches knowledge base articles and offers no other options?
Yes it is possible for Power Apps Portal - you need to update some site settings docs.microsoft.com/.../search
After making any changes you need to reindex the Search <yourportalurl>/_services/about (if you don't already indexed/cached values will still get found).
Think the main one is Search/Filters, but you may also need to update Search/Query and Search/RecordTypesFacetsEntities.