Uhh... I have a way, but it is a bit crude.
First, does the ward is the closest unit to the creep? Creep usually attemp to attack the closest unit, so if the ward is the closest, it will be a primary target.
A way (really crude) to solve this is check if a creep attemp to attack the ward(Unit - Unit is attacked), you order them to attack the summoning hero or the closest hero instead(Unit -Order with target unit) if there are hero in an area around that.
If you don't want the ward to get attacked, give it the invurenable ability, though it will confuse the creep a lot, since they will try to run away from a invurenable enemy first.