A custody sergeant who failed to act when a detainee slipped in his own urine and hit his head "abused his position", a misconduct panel ruled.
Instead of getting help, Gareth Starr laughed with junior colleagues and called him names, the hearing was told.
The panel said Mr Starr, who had 27 years' service, would have been dismissed if he had not already retired from Avon and Somerset Police.
It found his actions amounted to gross misconduct.
It added it was concerned junior colleagues may have perceived Mr Starr's behaviour as acceptable.
Mr Starr did not attend the hearing and did not dispute the allegations against him, the Local Democracy Reporting Service (LDRS) said.
Representing the force, Robert Talalay said the man in custody, referred to as Mr F, was "plainly drunk" when he was taken to Bridgwater police station in February last year after being arrested for assault.
He said Mr F had "staggered around" his cell before urinating on the floor and defecating against the cell door.
"He slipped, probably in his own excretia, falling several times onto his back or all fours," Mr Talalay said.
"Mr Starr and other officers were viewing. A number of them can be heard laughing, particularly when Mr F slips. Mr F slips for a fourth time and hits his head on the hard floor."
Mr Talalay said Mr Starr "actively discouraged" another officer from going into the cell, describing Mr F's behaviour as "amusing".
Police Federation representative Mark Loker suggested Mr Starr did not send officers into the cell because he was worried the "biological threat" could be used against them.
Chair Stephanie Beazley said: "PS Starr completely abused his position as someone tasked with looking after another human being.
"The only sanction that will go some way to repairing trust in the police that's been eroded by this incident is dismissal. Had he still been serving, PS Starr would have been dismissed without notice."