United launched an investigation after Northwestern University associate chaplain Tahera Ahmad complained about the incident last week aboard a flight from Chicago to Washington that was operated by Shuttle America for United.
Ahmad said in a Facebook post the flight attendant had discriminated against her when the attendant refused her request for an unopened can of Diet Coke, saying it could be used as a weapon, yet gave the man sitting next to her an unopened can of beer.
When she complained, Ahmad said, other passengers on the flight directed anti-Islamic comments at her.
Bob Birge, a spokesman for Republic Airways Holdings, which operated the Shuttle America flight on behalf of United, has said the airline's beverage policy does not prohibit serving unopened cans to passengers.
"While United did not operate the flight, Ms Ahmad was our customer and we apologize to her for what occurred on the flight," United Airlines said in a statement on Twitter.
"After investigating this matter, United has ensured that the flight attendant, a Shuttle America employee, will no longer serve United customers."
The airline said all its employees who deal with customers undergo cultural awareness training at least once a year. It also said Shuttle America employees who work with customers undergo cultural sensitivity training, but did not specify how often.
"United does not tolerate behavior that is discriminatory - or that appears to be discriminatory - against our customers or employees," the airline said.
Ahmad's description of the incident touched off a social media firestorm, including a Twitter campaign at #UnitedforTahera, which garnered global support and a call by many to boycott the airline.
Ahmad did not immediately respond to a request for comment on Wednesday.