iam running a mesh with 3 R510 AP with Unleashed Firmware 188.8.131.52.195.
There should be an AP-Group Option for configuring 11n only Mode.
But there is no such Option. This one is missing.
You can prevent b clients/traffic if you set the min-bssrate to 12. The previous answers to set n only and ofdm only are also accurate and huge benefits to wireless performance.