You can make PR/Res. as Never, so that PR / SRN will not be created.
IMG - PM & CS - Maintenance & Service Processing - Maintenance & Service Order - Functions & Settings - Define Change documents, Collective Purchase requisition, MRP relevance
Either at order level as well, you can make PR/Res. as Never.
But, it will enable you to calculate the Planned cost only. Unless you do Goods Receipt or Goods Issue for the components, actual cost will not be booked.
Without MM module, either Service or Components actual cost can't be booked.