I think that net0! is a default imposed by either CalibrePEX or your design kit.
Now, since you're not posting comands from a Calibre deck, I'm guessing you're using the Calibre GUI. So, go to PEX Options > Netlist > Format. The option right at the top of the tab should be "Ground node name". The tooltip specifically talks about intrinsic capacitances being grounded to that node, so you can pick the one you want instead of net0!.
If you're using a bulk silicon process, I'd also suggest giving the next option, "Ground layer name" a glance in the Calibre documentation. It turns into PEX GROUND LAYER in the output Calibre deck. It's useful for partioning your extracted intrinsic capacitance between the P substrate and the N-wells.