Passport 8600 STP Problem

[Numptie]

I have built a network consisting of 3 core 8600 switches connected in a ring with a further 8 8600 switches dual homed to two of the core switches. All of the 8600s are connected using single port VLANs with routing interfaces and OSPF enabled as the routing protocol. Users connect to 450T or BPS switches which in turn connect to single port VLANs on an 8600 switch. All VLANs were left in STG1(as STP not required) and STP is disabled at the port level but enabled at the group level on every 8600. The problem is that if a 450T/BPS is accidently connected to an 8600 with STP enabled the whole network dies within seconds. Why do the STP BDPUs not get dropped at the 8600 port the 450T is connecting to? Is the solution to put all 8600 interconnecting ports into STG2? I have tried disabling STP at the group level to no effect.