you do realise that the disc is held in place by a tiny little nut that pokes out as a locator when you go to mount the alloy wheel, its tiny like 10mm that holds it in place. maybe a couple of screws on some models at best but that is all. the disc is held onto the hub itself by the wheel bolts its just located using tiny bolt.
I get the impression you are trying to undo the big *** nut in the middle and thats drive shaft not disc.
the little mounting black pokey bolt thing is offset enough on the disc in the wheel studd pattern to not really cause an issue if you get her from the right angle.
if it keeps spinning after that put the fecker in gear, it shouldn't be tight enough that you do any damage to anything.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso