Starting from version 0.99 Monotone's disapprove command understands a revision range in addition to a single revision.
This means that you can disapprove bigger group of changes at once.
Of course, there are some limitations on the group of revisions. For example, there cannot be merge revisions inside the range.
This was my first bigger change to Monotone's code and along with that I gained commit access to monotone.ca, yay!.