A man who posted abusive messages on memorial websites dedicated to dead children was jailed for 18 weeks yesterday, and banned from using social networking sites for five years.

Sean Duffy, 25, admitted posting images on Facebook and YouTube mocking the deaths of four children, including Natasha MacBryde, 15, who committed suicide and was found dead on a railway line near her home in Worcestershire in February.

Reading Magistrates' Court heard that following her death, Duffy posted a video with Natasha's face imposed on Thomas the Tank Engine to a Facebook tribute page set up by her brother.

In a statement read to the court, her father Andrew MacBryde said that Duffy's actions had "added to the horror of dealing with the death of their beautiful daughter". Duffy, who had posted the images using false details, was traced by police through information from his internet service provider and arrested.

He admitted to "trolling", a term used to describe the trend of anonymously seeking to provoke outrage by posting insults and abuse online. As well are targeting Natasha MacBryde, the court heard that Duffy posted pictures on Facebook of car crash victim Hayley Bates, 16, of Stoke-on-Trent, who died in September last year. Duffy produced an image of Hayley with crosses on her eyes and red marks on her face.

Duffy also posted a message on an online memorial page to 14-year-old Lauren Drew, who died after suffering an epileptic seizure at her home in Coney Hill, Gloucestershire, in January.

The message, posted on Mother's Day, read "help me mummy, it's hot in hell". The court was told a friend of Lauren's had attempted a drug overdose after wrongly being blamed for the post.

Outside court, Lauren's father Mark Drew said: "Lauren didn't deserve this. Seeing him in court was really hard. I was so angry. This person hid behind the computer screen with no feeling."

Lance Whiteford, mitigating, said Duffy had been diagnosed with Asperger's syndrome at an early age. He said one of the characteristics was an inability to judge the reaction of others. Duffy had struggled with alcohol problems and lived "a miserable existence", he said, adding that although he had one caution for a similar offence, he had no previous convictions. As well as an 18-week jail sentence, Duffy was given a five-year anti-social behaviour order which bans him from social networking sites and prevents him buying devices which allow him access to the internet without police permission.

Anti-bullying charity Beatbullying called the conviction "a monumental move towards bullying and cyberbullying being taken more seriously". Sherry Adhami, a spokeswoman for the charity, said the sentence "sends a strong message... that bullying, whether online or offline, is not going to be tolerated."