The Hollywood Reporter said the former Beatles band member's star outside the Capitol Records tower was scribbled on during the weekend.
The graffiti included a spotted toadstool, a smiley-face, and the phrases -- "I love you" and "Blackbird... Rain was here."
"Morons did it," Gillian Lomax, host of the radio program "Breakfast with the Beatles," told the entertainment industry trade newspaper. "Rather tacky. There was a group of them, judging from the different colored pens. I tried to rub it out, but to no avail."
Lomax said she discovered the vandalism last Saturday while she was leading her "A Magical History Tour" group to Lennon's star.
Ana Martinez of the Hollywood Walk of Fame organization had workers out cleaning the star first thing Monday morning, THR said.
"We don't mess around," Martinez told the newspaper. "I think Capitol is looking into seeing if there's any video [of the crime.] They're registered state landmarks. Those are my babies! We've had people destroy stars, cracking or prying out parts of the stars, and they do go to jail and they have to pay back for the repairs. We just had one recently damaged -- Arsenio Hall's, and a couple near him."
Lennon was shot and killed by a deranged fan in New York Dec. 8, 1980. He would have turned 73 Wednesday.