[Jmenschenfresser]

Perhaps if the Cease Fire carried a mandatory ceasing of hostilities. Two ways to do this.

If made for 3 turns, you have a non-aggression for three turns and are unable to break it during that time. After three turns the status returns to none or war. But as someone pointed out, it could be used to get inside a person's empire.

Or you could make it so that the two empires couldn't even interact. Meaning they can't move into the same sector. Then all you have to do is park a ship over the WP and the enemy can't get into your empire.