At least the sentiment if not the exact phrasing pre-dates Medgar Evers (who lived from 1925 to 1963).
From a book titled The Russian Pendulum by Arthur Bullard published by Macmillan in 1919,
The armies of Tchaikovsky, Denikin, and Kolchak may enter Moscow and hang a few hundred of the Bolshiviki, but that will not kill the idea.
Or from Life magazine in 1951,
The fact is that he was out to destroy the ideas that Churchill stood for. He has told me many times: it is ideas he wants to kill, not men, but in order to kill the idea it is necessary to attack the man.
The Dublin Review (Page 260) Nicholas Patrick Wiseman - 1845
You may kill a few individuals, but you cannot kill an idea. An idea is immortal. In the midst of tempests an idea attains the stature of a giant, and like a diamond shines with a new light at every repercussion.
The Mining Congress Journal (Page 446) - 1920
These thirty-seven murders are the natural result of such sentiments as have been expressed by the sensational press in defining its attitude toward so- called radicalism. Their slogan has been "You cannot kill an idea."
Vanity Fair (Page 20) - 1915
You may kill a man, but you cannot kill an idea, and nationality is the most vital sort of idea.