Recently I had to prevent people automatically roaming onto a certain access point being used for development purposes. Hence, a MAC blacklist was put in place to force devices to fall back to another AP.

In order to add the blacklist, you have to add a couple of lines to your wireless configuration. This is done by editing /etc/config/wireless on the OpenWRT box.

vi /etc/config/wireless

From here, append the following lines into the config wifi-iface section:

option macfilter 'deny'
option maclist 'xx:xx:xx:xx:xx:xx yy:yy:yy:yy:yy:yy'

Where deny is the MAC filter operation policy. i.e. deny the following MACs from joining.
Another valid option here is allow if you wish to create a whitelist.

The maclist option is a space-delimited list of MAC addresses that you wish to block from joining your AP.

Now, simply reload the wifi config with the following command to put it into action.