u want to hide only the health bar or the whole unit?If u want to hide the whole unit write .mdl in the model option,if u want to hide only the HP bar then i don't know.
The health bar's size depends on the unit's selection scale, which you can set in object editor. However, using 0 can lead to glitches, spawning a health bar here and there now and then.
You can partly remove Locust, see here. Search locust/bar.
they are invulnerable so that's why they don't need health bars....
Set the selection scale of the unit to -.01, the units health bar would only be a mire 3 pixel hight and 1 pixel length (about) and black so it shouldn't be to noticeable.