At least two different sites repeat the same claim, that he did have the tattoo, and removed it with a knife. From an article on historynet.com, FROM GERMAN WAFFEN SS TO AMERICAN GREEN BERET:
In 1950, Törni moved to a Finnish community in Venezuela. He got a job
on a freighter carrying ore to U.S. ports on the Gulf Coast. Once the
freighter was in Mobile Bay, Alabama, Törni jumped ship—literally, by
diving into the water and swimming ashore. He wanted to get to America
and once on land would figure out how to stay there, a goal
complicated by the fact that he didn’t speak English. (Over time he
did learn the language but occasionally struggled with some of its
complexities.)
Törni reached out to Alpo Marttinen, who had been one of the most
highly decorated Finnish officers in World War II and was now in the
U.S. Army. Marttinen worked behind the scenes to get Finns to the
United States and serve with him under a new flag. Informally, they
were known as Marttinen’s Men.
William “Wild Bill” Donovan, the founding director of the CIA’s
predecessor, the Office of Strategic Services, in 1942 and now a
partner in a law firm, was approached by Finnish American leaders to
help their countryman, who had been arrested by the FBI for entering
the United States illegally. Donovan used his connections to get Törni
released.
However, the Finn’s Nazi past was still an issue. Törni used a knife
to cut out a piece of his left arm that bore a Waffen SS tattoo
indicating his blood type. The U.S. Immigration and Naturalization
Service was starting the deportation process when Donovan interceded
again. This time his law firm lobbied Congress to pass a bill that
would grant the Finn legal status. The legislation, signed by
President Dwight D. Eisenhower on Aug. 12, 1953, stated that Törni was
“considered to be lawfully admitted to the United States for permanent
residence.”
The above article mentions a 2008 biography, Born a Soldier: The Times and Life of Larry A. Thorne, by J. Michael Cleverley, which may provide you with more information (unfortunately no preview pages on Google)
Another book you may find interesting, from 1998, Soldier Under Three Flags: Exploits of Special Forces' Captain Larry A. Thorne, By H. A. Gill, Henry A. Gill. (This book does have some preview pages, so you may be able to gather some information here)
The knife claim is also repeated anecdotally on another site here, but this is basically a discussion forum type site, similar to the one you mention in the question.
