This is dependent upon which XBee module you are referring to and which ACK you are referring to. If you are referring to the ACK on an API transmit request, then Killer32 is correct. Setting the packet ID to 0x00 would keep you from getting an API TX status frame. But if you are referring to an ACK over the air, the only product that can do that is the 802.15.4 modules. Just configure the 802.15.4 modules with the MM command and you can disable the ACK over the air.