First of all, anyone who tried to play the first HandBall knows that the mouse controls are completely broken, sometimes unresponsive, sometimes extremely sensible.

The problem is: it's not possible to play a single-player match with the keyboard IF the DOSBox emulates the mouse device, which it always does, even if you disconnect your mouse device.

I found a solution:

1. Download "mouseoff.zip" from the VOGONS forums:
https://www.vogons.org/viewtopic.php?p=143507&highlight=mouse#143507

2. Unzip MOUSEOFF.COM and drop the file into your HardBall directory:
Default path is: "C:\GOG Games\Hardball\GameData\C\Hardball"

3. Open your "hardball.conf" file, insert the command MOUSEOFF.COM just one line above ITALSOFT.EXE and save it.
Default path is: "C:\GOG Games\Hardball\GameData"

4. Play the game.

5. DOS/HardBall will not recognize the mouse and it will only show keyboard controls for both players. You won't need a manual to discover the controls, because they are shown on-screen during gameplay.

Enjoy.