Meanwhile, the IRS is continuing to tighten its controls. In its September report, the Inspector General said that the efforts are helping but that more work needs to be done. For example, the report said, the IRS still has not taken action to prevent multiple tax refunds from being deposited to the same bank account.
"This continues to provide identity thieves with an easy method to obtain fraudulent tax refunds," the report said.
The new restrictions on the Master Death File could help close a significant loophole, though they come too late for the McClungs. Nonetheless, Terry McClung, who testified before the Senate Finance Committee on the issue in 2011, said he and his wife are gratified by the change.
"We're honored to have had any kind of influence, however small it might have been, to make this possible," he said in an email. "While we'll never know who stole Kaitlyn's identity, this is finally one small form of closure for this case."
—By CNBC's Scott Cohn. Follow him on Twitter
—CNBC News Producer Michael Tomaso contributed to this report.