How can I reverse a batch that I have posted inadvertently?

There is no specific batch reversal process.

You will need to perform one of the following processes:

1. Cancel the lines from the job (this is a job-by-job process where you set the status of the joblines on the Job Line Processing tab to a status of Cancelled)
   OR
2. You could re-post the batch with negative quantities to net off the impact of the transactions. At some point though, the combined negative and positive lines will need to either be included in an invoice or written off. 

If you are not able to add negative lines, there is a database level setting that you can set to allow these transactions to process.

UPDATE ENPRISE_JOBCOST_SETUP
SET ALLOWNEGATIVEENTRY='Y'