Is it possible to limit bandwidth for specific client?

Hey guys,

I have an Asus N66U router and I've been using the Merlin build which is great. I've been trying to find an option to limit the bandwidth specific clients on the Wifi or for the "Guest" access. We have a tenant sharing our internet and I'd rather not let him access the full bandwidth so that we don't go over. The best I've found is playing with the QoS rules but that doesn't apply it only to a specifc IP or the guest network.

Is something like this possible?

Thanks,

Flaviu