If the subcontract line has been added in error and is not required then do the following:
Go to the Workshop Reception and select Sub Contracts - find the subcontract in question and post it at zero cost.
On the workshop job - for the subcontract line, bill it against an Internal Account and change the value to zero.
This will generate a zero invoice.
If the subcontract line needs reversing and the job cancelling, then do the following:
On the workshop job, raise another sub-contract line for a credit of the original value.
Go to the Workshop Reception and select Sub Contracts - find the subcontract in question and post the subcontract at the credit value to the original supplier account.
On the workshop job - for the subcontract line, bill it against an Internal Account and change the values to zero.
This will generate a zero invoice and the job can be released.
You cannot cancel jobs that have subcontracts against them.