The family of Roger Sylvester say they hope the inquest into his death will lead to changes in the way police officers restrain people in custody.
Roger Sylvester's family want lessons to be learned
The relatives are also considering legal action against the Metropolitan Police officers involved, after the inquest ruled Mr Sylvester had been unlawfully killed.
Mr Sylvester's cousin, Shirley Sylvester, said: "Roger was restrained dangerously, that was what the jury concluded.
"They said it was unlawful and we agree."
The family believe Mr Sylvester was held on his stomach - which increases the risk of suffocation - although the officers say he was restrained lying on his side.
"Although there are issues about how long one can be kept in the prone position, it is something that we feel very strongly about," said Ms Sylvester.
"I want their [the police] behaviour to be changed. Training is just one side of the coin and there has to be appropriate sanctions because that is part of this issue."
Many family members, friends and campaigners were in court to hear the verdict, which was greeted with claps and cheers.
A number of relatives dissolved into tears, and later the deeply-religious family held a prayer outside the court.
Mr Sylvester's mother Sheila, 68, a retired nurse, said: "I'm very pleased at how it went today. I thank God and thank the campaign. It is been four years and nine months of hard work.
"We have had a lot to put up with and I have a lot to think about now."
Mr Sylvester was restrained by police after being found naked and behaving "bizarrely" outside his own flat in Tottenham, north London, in January 1999.
He was taken to hospital where he was restrained again but stopped breathing and fell into a coma from which he never recovered.
The family said it was considering whether to bring a civil prosecution against the officers involved.
But it hoped the Crown Prosecution Service would review its decision not to bring charges against any of the officers.
Mr Sylvester's brother, Bernard Renwick, said: "I hope that the CPS takes a good strong look at the evidence in this case."
Ms Sylvester said the family had to discuss its next legal steps with its lawyers.
"There needs to be a reassessment. Lessons have not been learned," she said.
The officers involved released a statement saying they were also looking at the legal options - believed to include a possible judicial review of the verdict.
And the Metropolitan Police force as a whole said it took Mr Sylvester's death "extremely seriously" and was working on ways to reduce deaths in custody.