Any competition should be friendly first of all, even though depending on how local haunts are designed or operated can impact business between haunts.
I also know some haunt owners can be a real peach and cause nothing but trouble, but it's important to stand tall on your own ground and be the better business men or woman by not sinking to their level.
Would also be a nice gesture to ask permission to do such a thing, but if that's something they did on their own then I would say that there might not be anything you can do about it.
It really makes things hard when you know every haunter strives for business and to be different or the best, we are all pationate and work hard on our haunts year in and year out, but if there is bad blood between one business to another it just hurts the industry. It never does any good to retaliate and that's why I said;
"BE THE BETTER PERSON EGNOR IT AND MOVE ON" There are more important things to worry about then a sign and again I am sorry I know that there is a lot of pride that goes into each haunt but keep to your self. Focus on your business and nothing else it's that simple.
Oh and one more thing, it's no different then a Target opening across the street from a Walmart.