How to tell MS no channel is available?

Hello guys, Suppose I have 8 MS registered on my nanoBTS and the ninth wants also to register. How can I tell the ninth MS "Sorry dude, there is no room for you, try another ARFCN"? I've checked the REJECT causes in gsm 04.08, but couldn't really find one which is informing the BTS is out of free channels. Now my mobile keeps trying to register to my nanoBTS, although I rejected him with a standard cause. Maybe someone can give me a hint where to look at? Thanks.