According to Wikipedia[1], the first use of the term 'space marine' in literature was the 1932 short story Captain Brink of the Space Marines by Bob Olsen. That's a full 43 years before Games Workshop was founded, and 55 years before Warhammer 40K was introduced. And the prior usage of the term and concept kept coming strong throughout the intervening half-century. 1: http://en.wikipedia.org/wiki/Space_marine
So why is this not an open and shut case ?
"Coke" has other meanings http://en.wikipedia.org/wiki/Coke_(fuel) and yet is still trademarked.