The easiest way to get AutoCAD (and by extension Civil 3D) to not calculate something is to freeze the layer. In this case, just freeze the layer the grading objects reside on. For the NCS layer standard, that is typically C-TOPO-GRAD.
You can edit the block with the REFEDIT command or The BEDIT command. From there you can make changes to the block, then save and exit the editor to update the changes to the block.