The same way you prevent the caster from readying an action to mindrape as soon as the creature appears. If you start conspiring to attack your called creatures, the contract says that shit happens to you.
Well, it depends on how you're defining "contract". If the creature can't make a contract with you because you mindrape it, then who cares?
How about this: when you cast the spell, until the contract has been agreed upon, (a) you can't cast any more spells on the creature, (b) no one else can see or interact with it, and (c) the creature can't leave or make actions through the circle.
Possible option, take a leaf from Tome of Magic.
When you Call up a creature, it appears in image and spirit only. There is nothing to affect, except by your words. Once contract is
established, then the Outsider actually appears physically, and the contract is binding both ways, in that it must fulfill the task assigned, and you MUST establish a penalty for pact breaking(as would have been done with a real contract) on each party.
Now add to that, if the Outsider is killed in the process, it eventually reforms on it's native plane in a new body after a time.
For necromancy, I got an idea.
What if necromancy works JUST like the metamorphosis option? You animate a corpse, which creates an Undead Creature, with traits you can pick from based on the creature used. Any trait you didn't pick rots off in the transition.