PHOENIX—When it comes to marking up historic signs, good grammar is a bad defense.

Two self-styled vigilantes against typos who defaced a more than 60-year-old, hand-painted sign at Grand Canyon National Park were sentenced to probation and banned from national parks for a year.

Jeff Deck and Benjamin Herson pleaded guilty Aug. 11 for the damage done March 28 at the park’s Desert View Watchtower. The sign was made by Mary Elizabeth Jane Colter, the architect who designed the rustic 1930s watchtower and other Grand Canyon-area landmarks.

Deck and Herson, both 28, toured the United States this spring, wiping out errors on government and private signs. They were interviewed by NPR and the Chicago Tribune, which called them “a pair of Kerouacs armed with Sharpies and erasers and righteous indignation.”

An affidavit by National Park Service agent Christopher A. Smith said investigators learned of the vandalism from an Internet site operated by Deck on behalf of the Typo Eradication Advancement League, or TEAL.

Authorities said a diary written by Deck reported that while visiting the watchtower, he and Herson “discovered a hand-rendered sign inside that, I regret to report, contained a few errors.”

The fiberboard sign has yellow lettering with a black background. Deck wrote that they used a marker to cover an erroneous apostrophe, put the apostrophe in its proper place with white-out and added a comma.

The misspelled word “emense” was not fixed, Deck wrote, because “I was reluctant to disfigure the sign any further. ... Still, I think I shall be haunted by that perversity, emense, in my train-whistle-blighted dreams tonight.”

Deck, of Somerville, Mass., and Herson, of Virginia Beach, Va., pleaded guilty to conspiracy to vandalize government property.

The TEAL Web site now has only this message—“Statement on the signage of our National Parks and public lands to come” —without a period.