How come you want your niki back to orginal? i would keep HardSpl just incase i wanted to flash again.
On the post itself there should be a bit telling you how to remove it. It is possible though i remember seeing a removal tutorial for my old Wizard.
02 XDA Mini S (Wizard G4)
OS: 5.2.1993 (build 185188.8.131.52) TNT WM6 Rom
HTC Touch Dual (A.K.A Niki)
OS: 5.2.19716 (build 197184.108.40.206) WM6.1 Tom_Codon Niki Project