I have had this problem as well, the documentation says that should work however i have never been able to get it to work like that on my rigs.

However what might help is if you want to disable 0 if you set --opencl-device 1 then set the number of threads to 1 less than the number gpus it will start with 1 and then not use 0.  At least it does on my machine, but if you are trying to lock out any other except 0 I don't know any way to do it.