I have here few solutions, set the minimum movement speed from game constants to zero:
1. Set the movement speed of the unit to 1, however if the game is long and the unit is still in the map, you will notice that the unit has moved a few distance from his starting position.
2. Add an endurance aura with a -1000 movement speed bonus and make the targets allowed to self only, add that ability to the unit and make sure that the movement speed base of the unit is greater than 0. However, the buff from the endurance aura is annoying.