The British athlete has demanded that the football club remove her name from the stand in Bramhall Lane if Ched Evans is offered a new contract.
One troll said: "I hope he (Evans) rapes her."
The club have said they were "appalled" by the Twitter taunts being aimed at their patrons and supporters and have pledged to cooperate with South Yorkshire Police who are investigating the abuse.
In a statement, the club said: "Sheffield United Football Club is appalled at the recent condemnation and abuse that has been levelled at both some of its more high-profile supporters and those who have no connections to the club but who have voiced their opinion on the matter.
"Regarding the personal tirade against individuals, the club will work with both South Yorkshire Police and any other supportive body to help identify the instigators. This will include abuse through social media messaging and chanting at grounds.
"Anyone who is found to be involved in such practice will be banned for life from Bramall Lane. Sheffield United will not accept the abuse that is being received by those who have voiced their opinion. They have felt it necessary and have been courageous enough to have made their statement and this should be respected."
Wales international Evans was jailed in April 2012 for raping a 19-year-old woman in a hotel room in Rhyl, North Wales. The striker denied the allegations of rape, claiming the sex was consensual, but he was found guilty by a jury at Caernarfon Crown Court.
He was released from prison last month after serving half of the five-year sentence.
The 25-year-old was due to begin training with the football club this week after his release from jail last month, but following the public furore he has been branded "toxic. "
A source revealed that Sheffield United are preparing to rule out his return.
TV presenter Charlie Webster, Sixties pop star Dave Berry and Sheffield businesswoman Lindsay Graham all resigned as patrons of the club since Evans returned to training.
Deputy Prime Minister Nick Clegg - a Sheffield MP condemned the "sick and vile rape threats" made against Ennis-Hill.
An appeal against Evans conviction was rejected by three judges at the Court of Appeal in 2012.
His case is due to be examined by the Criminal Cases Review Commission.