The shop window of a pop-up gallery in Ljubljana that showcases works by street artist Banksy has been vandalised with what the exhibition's curator has described as the crudest type of graffiti.

While graffiti artists all over the world destroy and paint over each other's work, this particular act was not a case of a graffiti writer with something to say or a new piece of art to make, the gallery said on 11 March.

The curator David Rjazancev described the author of the graffiti, which appeared at the weekend, as a "charlatan who wrote his name over Banksy" in what is known as tagging.

"If there is any ranking of graffiti quality, tagging is the absolute bottom of that scale. Some artists have taken tagging to a higher level, even calligraphy. However, we unfortunately had a visit by a common vandal who made a mess that has no artistic value or message," the curator was quoted as saying.

He would not name the perpetrator, but said he was known in the Ljubljana graffiti scene as a "hooligan who is incapable of making a good graffiti or even addressing an important social or personal issue, something that Banksy and many other graffiti artists do".

Mounted by the Deva Puri Gallery, the Banksy in Ljubljana exhibition is on show at a former sports footwear shop in Ljubljana's Čopova street until 21 April. It features more than 100 works by Banksy and artists who have inspired him or collaborated with him.

The gallery is planning to open soon a new space inside the exhibition aimed at local graffiti artists. "In this way, the wider local public will be able to get to know their work and learn to distinguish art from vandalism," the gallery said.