An inquest jury found last year that Ibrahima Sey, 29, was unlawfully killed at Ilford police station, north-east London, two years ago.
But the CPS said there was still conflicting evidence over the exact cause of Mr Sey's death, which meant there was "no realistic prospect" of convicting any police officers.
Last night Mr Sey's widow, Amie, said: "I am deeply disappointed by the decision of the CPS. All I want is justice to be done and peace for my family."
Metropolitan Police officers arrested Gambian-born Mr Sey, who had mental health problems, in March 1996, after he made threats to kill his wife.
He was taken to the police station where a struggle broke out. Officers restrained him by handcuffing his hands behind his back, forcing him to lie face down on the floor and squirting the spray in his face.
Last October, a jury heard evidence that Mr Sey had died of restraint asphyxia and excited delirium, prompting the CPS to reopen the case and the coroner to call for police forces to review their use of CS spray.
But the CPS said yesterday that there were still "conflicting and inconclusive" views among experts over whether excited delirium could cause death on its own, without an element of restraint.
It said in a statement that Hertfordshire Police, working with a senior CPS lawyer, had undertaken "further exhaustive" inquiries since the inquest in both Europe and the United States.
"In the light of all the evidence now available, it is not possible to be certain of the cause of death. Therefore there is insufficient evidence to justify proceedings against any police officer," the statement said.
The Government last week announced a safety review of CS spray amid mounting concern over its use by police.
Mr Sey's family is meeting lawyers to consider what options they have for any further legal action.Reuse content