Disabling the z axis on the Repulsion Field ability!

OK..so once again I'm designing some sins modes and next on my last is a fast but fun "SINS SOCCER" mod.

Principle is simple.

3 races

4 types of ships

-Destroyer: Very expensive, its job is to destroy opponents players

-Striker (fast move but weak repulsion)

-Central (average for both)

-Defender (slow speed but storng repulsion)

2 Homeworlds: 1 for each player with INCOME in order to recreate more ships

1 goal: To be designed...it will be located in a centre area such as an asteroid field as to be located as close to the centre as possible!

1 Ball: A modified cap ship with no speed, weapons...NOTHING! A simple ball meant to be pushed around.

Question is...how to disable the Z axis so that when the ships use repulsion field to "kick" the ball, the ship doesnt start flying upwards!!??

If possible?