Limit Bandwidth Of All Computer (regardless of ip/MAC)

I wanted to know if it is possible to limit bandwidth of all computers connected to a router, regardless of the ip/MAC address. I do not want to limit the total bandwidth of all user but individual bandwidth of all user.

More info: I am small business owner. I have a small coffee shop. To compete with the bigger guys we are offering free wifi. We are having a problem, sometimes one person slows down the wifi for everyone. 

I want to limit the maximum download and upload speed of each user to 3 mbps/1mbps. I have looked into Qos, and I find it too complicated. Besides I don't care if someone is using p2p or watching movies as long as their download speed is below 3 mbps. I cannot use the feature of assigning static ip address/ limiting by MAC address because new customers are coming and going.

I am looking for a feature in firmware that allows you to limit the maximum download/upload speed all ip addresses connected to the router. (may be with an exception of ip addresses that this does not apply to.)
I don't want to have to enter in a limit for each ip, but have it be done by the router.
I would prefer to just have this running on the router, without the need for dedicated computer attached to the router.

It would be awesome if someone could let me know of a firmware that would allow be to do this. I don't mind buy a new router, if a can get a firmware that will allow me to do this. My budget is $200 though.

I tried to talk to linksys support and they told me to buy a commercial router that is more than a thousand bucks. I cannot afford that. I am really hoping someone here can help me out. Thanks in advance.