Instead of a graffitti party, could you do a fingerpainting social instead? We did ours outside in the quad (a big, flat grassy strip of land on campus). That way, we didn't have to worry about being messy with the paint, or have to deal with any decorations.
The fraternity guys took their grills out there and grilled burgers and hot dogs for us. We brought the chips, non-alcoholic drinks, and condiments. It was a lot of fun, no alcohol was involved, and -- best of all -- other students passing by the quad on campus got to see us in action, which was great for our PR.
Good luck!