Viewing the bandwidth usage of a wireless device connected to my router

It seem that tomato with iptables can be used as bandwith monitor. detailed instructions

I do not understand why you cannot selectively block clients from the network, until you empirically find the guilty one.

If you are looking for a more high-tech method, you may try using a packet sniffer, also called packet analyzer, to analyze all traffic on the network from your computer instead of from your router.

A packet sniffer can capture packet information within its subnet, even if these packets are not addressed to your computer. The feasibility of this method depends on the behavior of the router and whether it broadcasts all network packets or not. Wireless networks are of course much easier to snif than wired.

See this article, Free Packet Sniffer Software, for a list of such products (not all are available for Linux). The best known sniffer is Wireshark.

See also this article : Sniffing Traffic on Your Home Router or Hub.