Reading more about the Rigidbody 2D in the excellent documentation, I found the Linear Drag property. Setting it to 10 (default was 0) fixes the problem nicely.
This makes sense to me. Since I disabled gravity (by setting Gravity Scale to 0) as the tutorial suggests, there is nothing stopping Ruby from floating endlessly when she gets momentum. I think of it like an object in outer space that is missing any form of drag and therefore never comes to a halt. By simulating drag Ruby will still be pushed but comes to a halt shortly after the pushing force stops which makes for a nice effect of being pushed around by the robot.
I still wonder why I could not find a solution in the tutorial and why the problem is not mentioned in the mega thread (thanks @JeffDUnity3D for pointing me towards it).