Is it possible to batch-reject multiple invoices in one go?
This is possible. To do so, please go to Reports - Invoices - Batch invoicing processes and select the 'Approve Invoices/Credit Notes' option. Then select the billing office / customer / engagement the invoices belong to. From the actions menu, select 'View invoices/Credit notes'.
Now a window displays the invoices, select the ones to reject and then select Actions - Reject Selected.