Referee Eddie Wolstenholme brought an end to the game after it descended into farce with the Blades reduced to seven men.
Simon Tracey, Georges Santos and Patrick Suffo had earlier been dismissed, enabling the Baggies to establish a 3-0 lead through a Scott Dobie double and a Derek McInnes goal.
There had been an off-field disturbance early on when an Albion fan ran across the pitch and as police and stewards moved to prevent other supporters doing the same a stand-off developed.
A wall of police came across to the small section of the stand and the situation calmed down
All hell broke loose in the 65th minute when Santos, who had come on seconds earlier, clattered into Andy Johnson and was sent off. There was some history between the pair, Santos having his jaw broken by a swinging Johnson elbow while the Albion man was at Nottingham Forest.
In the skirmish that followed another substitute, Suffo, appeared to headbutt Moore and was also dismissed.
The game had degenerated into farce and United captain Keith Curle - who should have been setting the right example - was lucky to escape a red card after appearing to land punches at McInnes.
Michael Brown had already gone off in the 79th minute and Ullathorne's injury meant the game was stopped.
It is still unclear whether West Brom would be given the win - that is for the Football League to decide.
Michael Brown showed the first flash of inspiration when he embarked on a mazy run into the box in the fourth minute, beating two West Brom defenders before his left-foot shot was deflected wide.
United suffered a blow in the seventh minute when Simon Tracey was sent off after he raced from his goal and blocked Dobie's shot with his hands following Russell Hoult's long goal-kick.
Deputy stopper Wilko de Vogt came on at the expense of Peter Ndlovu, who had made a lively start to the game down the right.
Neil Clement's free-kick following Tracey's deliberate handball was blocked by the wall.
The Blades' resistance was short-lived as Albion took the lead after 17 minutes.
Dobie played a clever ball between United's centre-backs allowing Andy Johnson to race unhindered to the right touchline. He crossed to the six-yard box where Dobie, who had continued his run, scored his ninth goal of the season with a wonderful diving header from point-blank range.
United youngster Michael Tonge tried to restore parity but his 25th-minute free-kick from 25 yards flew just over the crossbar.
Seven minutes later a neat one-two between Paul Peschisolido and strike partner Laurent D'Jaffo allowed the Canadian through and he went down with Larus Sigurdsson close at hand, but the referee felt no foul had been committed.
The home side wasted a glorious chance six minutes later when a centre from Brown found Phil Jagielka in acres of space 10 yards out but the midfielder somehow contrived to nod the ball wide.
Clement tried to double his side's lead a minute before the break but his 20-yarder after he cut inside from the right was well held by de Vogt.
United started the second half more positively than the visitors and the first real shot on goal came from Brown in the 51st minute, whose low left-foot shot from 25 yards was held by Hoult at full stretch diving low to his left.
Albion added a second after 62 minutes. Igor Balis took a short corner and found Derek McInnes unmarked on the edge of the box and he smacked a right-foot shot into the top corner.
Three minutes later all hell let loose when Santos, who had come on seconds earlier, clattered into Johnson and was sent off.
There was some history between the pair, Santos having his jaw broken by a swinging Johnson elbow while the Albion man was at Nottingham Forest.
In the skirmish that followed another substitute, Suffo, appeared to headbutt Moore and was also dismissed.
The game had degenerated into farce and United captain Keith Curle - who should have been setting the right example - was lucky to escape a red card after appearing to land punches at McInnes.
A third goal for West Brom was inevitable and it came in the 77th minute when Moore headed a deep corner kick back across the goalline and Dobie pounced to make it 3-0.
Brown then left the field and when Ullathorne could not carry on Wolstenholme had no choice but to end the match.