As I am using Grow for a long time, I see that my History folder was growing on, without limit.

I have decided to quick patch growl in order to add a EnableHistory user settings, default to True but if set to "False", no file will be created in History folder.

Edit your configuration file (should be located at %LOCALAPPDATA%\Growl\2.0.0.0\user.config) and add the following lines:

<setting name="EnableHistory" serializeAs="String">
    <value>False</value>
</setting>

The SVN diff patch is included in the 7z archive beside to the installer.