Here's a list of important, mainly American, crime authors who've had only short story translations in Finland:
William Campbell Gault
Norbert Davis (see also this)
William G. Bogart (the WikiPedia link doesn't say that Bogart's hero Johnny Saxon was an ex-pulp writer himself)
Hugh B. Cave (see also this)
George C. Chesbro (two Mongo stories, if I remember correctly)
George Harmon Coxe (okay, he had some serials published in magazines)
Steve Fisher (I've been forgetting to link to this excellent post at Woody Haut's blog)
William Lindsay Gresham (you've all seen the film about his wife, haven't you?)
Harold Q. Masur (see also this and this)
Sam Merwin, Jr.
Frederick Nebel (see also this)
Francis M. Nevins, Jr.
William Nolan (see also this)
Lawrence Treat (see also this)
Robert Turner (see also this)
Leslie T. White
Does William Barrett count? Many of his Needle-Mike stories from Dime Detective were published in Finnish in the fourties, but none of his adventure and religious novels ever appeared here.
I also noticed that I was wrong about William Ard when I said that he has nothing translated in Finnish. He has a short story in a men's mag in the fifties, called "The Only Piece of Evidence" (or something to that effect).
By the way, Bart Spicer has been published in Finland only in Swedish!