I would like to create a rigid region of nodes. Is it possible?
To create a rigid region, you can use CERIG (you could also just make the modulus of those elements really high, but you may run into condition issues there). The attached simple input file demonstrates the idea. It is worth noting that you may want to delete the rigid elements before creating the rigid region, but leaving them in for the purpose of this example shows that the elements carry no load.
! I'll arbitrarily choose a region to be rigid
! then, I'll use CERIG to make it rigid
! you may want to delete the unneeded nodes elements
! before creating the rigid region, but they are left
! here to demonstrate that they carry no load