From a timing perspective, having them walk around the finish line may still result in the system seeing them and thus having to make sure they are not given a time. Procedurally, It may be easier for them to cross the line and then identify themselves to the lead timer right on the finish line so they can immediately mark them as a DNF in the system rather than wait for the report from the SAR team. But that is something that needs to be discussed with the finish line timing team, the aid station, and SAR.
Even if they DNF, they are still a registered runner and thus I feel they should be permitted to make use of the finish line aid station and medical tent if needed. The only thing that they are not permitted to accept are the finisher medals and finisher shirts.
As for Mr. Spandex, bandits are annoying and there is not much we can do about them short of having some police at the finish line arrest them for theft of services. Of course in lieu of jail time maybe we can get them to show up on Monday/Tuesday morning to help the ops team retrieve the hose from the summit. :-)