Dear Yael,
First of all, why the machine scrap is maintained as an activity? Are you incurring any additional cost, say like material handling cost etc on the same?
If not, you can take out the activity and user can declare this under the "Scrap" field while doing confirmation. This will not have quantity impact and will hit the costing as well.
The scrap is basically generated as part of production and this cost will be a minus entry against the production order. costing team can define a standard cost (based on market rate) and when declared under "Scrap", the same will be reflected against order.
If you do not want to remove the field technically but hide it from users, then you can do the same from OPK0, or set it in costing formula. Later on, if required, finance team can transfer the cost from cost center to this "machine scrap" activity type if they want.
Thanks
Prem