Long, 37, joined the club earlier this year to assist Tony Rea, but revealed on his Twitter page last week that he had left The Hive.
“Unfortunately, Sean’s performances as an assistant coach did not meet the professional levels expected at the club. As a result his position as full time assistant is no longer continuing.
“Whilst this decision had to be made by the club, we do wish Sean the best for the future.”
The Broncos have conceded 108 points in their opening two Super League games, defeats against Widnes and Salford.