Wow. It amazes me how immature and childish some people can be! I mean, spray painting a building?! C'mon now.
With what everyone else said - wear letters with PRIDE and show them just how strong our sisterhood really is! Keep your heads up - we know how awesome our organization is, and remember to be PROUD to be a Sigma no matter what.
I'd also try going to PHC and talking to some of the other Greek organizations. My guess is that someone is going to know something about what went on and who had a part in it.
You ladies keep your heads up. I know it's one thing to say and another to do, because as a Tri Sigma, I can't believe someone doing this, but don't give whoever did this the satisfaction of seeing you ladies down and upset about the incident.