Omit “former”....
Once a Marine, always a Marine.
Incorrect, once you've left active duty you're a former Marine and the prefix "ex" is appropriate as well for some like Ritter, Dellums, Carville, Garwood, Oswald, Lonetree, Dennehy, Murtha, Glenn, Farrell, McFarlane, Lehrer, Shields, etc.