The altered advertisement, which had said "Shoot a .50 caliber only $29," was taken down within hours and police began an investigation after the vandalism was reported, Officer Larry Hadfield said.
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An email statement from the activist group INDECLINE took credit for the protest.
A telephone caller who said he represents the group but refused to give his name provided the statement to The Associated Press.
It said the billboard action was to spur debate about "inadequate gun laws that are currently placing value on assault weapons over that of human life."
INDECLINE also took credit for putting statues of a naked Donald Trump in cities around the country in 2016.
The man said the group provided a video to KTNV-TV showing at least two people wearing dark clothing scaling the billboard and pasting strips on the sign with the words "School Kid" and "Defend Lives Reform Laws." A person climbing a ladder appeared to be wearing a wool mask.
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The billboard was not far from the site of the worst mass shooting in modern American history. The Oct. 1 attack killed 58 people.
The spokesman said the billboard action also took inspiration from the Feb. 14 school shooting that left 17 people dead in Parkland, Florida.
Florida sheriff gives timeline of events in school shooting
Employees at Battlefield Las Vegas, the gun range that invites tourists to shoot a semi-automatic rifle, referred questions to managers who did not immediately respond to telephone and email messages.
The billboard near Interstate 15 is about 3.5 miles from Mandalay Bay resort, where a gunman fired more than 1,000 shots from 32nd-floor windows into the crowd at an open air concert before killing himself.
SEE ALSO: Timeline of mass shooting on the Las Vegas strip
Timeline of Las Vegas deadly mass shooting