John Hall, 48, asked Paul Williamson if he could sing a song on the machine at the Mariners Arms pub in Liverpool on Sunday, March 3 this year.
The DJ said he was busy and told Hall, who is a self-employed roofer, to write down his name and song choice on a piece of paper and that he could sing later.
Instead Hall wrote a note saying “’f*** off b****nd” and passed that to Mr Williamson, who suffers from fibromyalgia, a neurological disorder which causes pain all over the body.
Throughout the evening he asked Mr Williamson, 58, if he could sing a song, the Liverpool Echo reports.
Iain Criddle, prosecuting, told Liverpool Crown Court: “Mr Williamson told him he wasn’t going about it the right way by writing notes such as that.”
In CCTV footage played to the court Mr Hall is seen approaching the machine and grabbing the microphone.
The DJ and other people asked him to stop, before he walked over and hit Mr Williamson on the head.
Mr Williamson fell and hit the back of his head on a shelf – before Hall punched him until he was dragged off.
When the shocking footage was played, Judge Brian Cummings, QC, winced and said “oof”.
Mr Williamson, who was dazed and bleeding from his head, was taken to hospital and given a CT scan.
He suffered a crescent-shaped cut to his right temple and a large bump behind his right ear.
Mr Williamson read a victim statement in court and showed the judge his scar, which he said was “a permanent reminder”.
The dad-of-one, who also has a heart condition, said he was a DJ “to keep me going” and get out of the house – not for financial gain.
He said: “This has had a huge negative impact on my life in respect of confidence.”
Mr Williamson said the attack affected his pre-existing anxiety, made him think about his age and he considered quitting DJing.
He said: “It just made me more aware of my disability and how vulnerable I’ve become – I used to be very strong.”
The victim added: “I’ve been left with a scar, but the little scar I’ve been left with is not really comparable to the mental scar.”
Hall had earlier admitted assault causing actual bodily harm.
Judge Cummings said it was a “very nasty” and serious example of the offence, which “might easily have been charged as wounding”.
He said Hall wouldn’t have known Mr Williamson was “particularly vulnerable“.
The judge said: “You are responsible for the very serious consequences that this offence had and continues to have.”
Hall was jailed for 12 months and banned from the pub for two years.
Judge Cummings said: “You cannot do what is shown on that CCTV footage to another person and expect not to go to prison.”