There is an mid-size art gallery on U Street (near 14th) that will occasionally open its doors for private parties called The Hamiltonian.  Blank walls and floor, but they will probably want to leave the art on the walls.  

Artomatic has a very large raw space and I know BrightestYoungThings.com used it for a party once.   The Space, where I had my re-launch party, is somewhat raw.  Oh, and if it’s for something edgy, there is a hidden spot called Fight Club, but it’s like port-a-potty raw.