Shouts of "Out Gerrard out" failed to get the leader to come out of the town hall to meet them.
WFRAG have, raised over £300,000 towards their campaign so far and have spent £10,000 in court costs.
The case will go before the High Court on July 13 at a cost of £250,000 a week and they are hoping it will only last a week.
The group received a setback this week when a High Court Judge agreed an application by the council to have the 31 Labour councillors discharged from the action. The ratepayers are, however, appealing against the ruling.
Coun Slack was chairman of the council's planning committee and vice-chairman o»f several others.
He told Mr Gerrard last night that he intended to remain a Labour Party member and would not support either of the opposition parties who hold 26 of the council's 57 seats.
Mr Gerrard said: "He said that he had reached the point where he couldn't carry on supporting the group and that it would be dishonest of him to continue.
"I'm sorry it has come to this but on this issue, he has been honest right the way through."
Coun Slack was replaced as planning chairman following the full council meeting by Coun Jonathan Brind.
Labour had a majority of three last night to reject Tory and Alliance motions calling for the legal action to be ended and pledged that the court case would be contested.