Hello. I play F3 since not that long, but discovered that awesome G.E.C.K yesteday only, seems to be amazing from here

I'm working on a Vault with rusted textures, but I need a wall to join 2 others 'blocks'. I can't find that wall alone, only from some others blocks where a floor or/and ceilings are inclued with the wall I want. My question is then, is there a way to remove some polygons of a block ? (so I can remove the floor or ceiling to keep only the wanted wall).
Sorry for my ignorance, I couldn't find any result using the search feature. If anyone could help me, that would be great