It depends. You can refuse the ETD before the TDC (where you submit the "papers, please"). There was a court case that said you can still be subject to civil penalties if you refuse the screening process after it's been started. This was because someone could theoretically test the limits and then walk away at a critical time in the screening process. Of course, this court case was long before the days of widespread patdowns, the war on water and NoS.

That being that, has anyone ever been fined from walking away? I don't think anything happened to the "If you touch my junk..." guy after he walked away.