ack: Excluding only one directory but keeping all others with the same name

The older versions of ack can only take the folder name, not the folder path. As of version 1.93_02, they’ve added this ability in:

1.93_02     Wed Oct  6 21:39:58 CDT 2010
   [ENHANCEMENTS]
   The --ignore-dir option now can ignore entire paths relative
   to your current directory.  Thanks to Nick Hooey.  For example:

       ack --ignore-dir=t/subsystem/test-data

(From betterthangrep.com/Changes)

You can check which version you have with --version:

ack --version