Exclude MAC addresses from radius

Is there a way to exclude PPPoE sessions from getting a 10.250… address when failing to connect?
I’ve got devices which are unable to set a Service-Name and therefor they always connect to the MikroTik-Splynx server instead of a 2nd server in the network with a different Service-Name.

Instead of handing out a 10.250.x.x addresses I’d like to see selected MAC addresses ignored and given the possiblity to be responded to by a 2nd PPPoE-server.

I’d like to hear your idea’s on this.