Someone ignores a message of a NAP and I consider open war the only option. Of course - if they aren't moving forces to attack me immediatly and another nation is I'll concentrate on the other nation first.
But I'll always immediatly move to war footing if someone turns down a NAP and I have no other threats. They aren't replying because they want a war - just not at this moment. So hence, it's to my advantage to start fighting with him ASAP unless I belive I'll be in a better position to attack him later. But make no mistake... we are at war
The other possiblity is that he just missed the message... but I'll never put my money on that.