I would approach the church pastor and explain the situation nicely to him/her. Ask if there is any way they could minimise or reduce the lighting or if it is even needed at all. A couple of smaller lights closer to the cross will still make it visible to passers by, but wouldn't light up the whole sky. They will also cost their parish less to run and replace.
Perhaps they could use directional shades to direct the light onto the cross only, not the whole sky.
As part of your argument, you could tell them that their lights are making it hard for everyone to see and enjoy the Lords creation up there.
Baz.