Using [harm] will prevent your char from auto selecting a new target. I use this on all of my slaves. e.g.

/cast [harm] Spell

Or using Ualaa's macro

/assist [target=party1target]
/castsequence [harm] !AutoShot, Steady Shot
/cast [harm] Kill Command
/script HideUIErrors()

This change will mean that if Party1 has a target the hunter will attack it. If it doesn't have a target then the hunter will do nothing. It will not grab a new target automatically.