NetLimiter 2 Pro can only monitor and rate limit, but it can't cap the usage.
I don't know of any client side applications that can place a cap on the usage either.
So you'll have to buy hardware or run some kind of router OS to cap the usage. If you simply want to cap HTTP usage, then you can most likely do that by running a transparent SQUID proxy server.
Otherwise, you'll have to run a router with DD-WRT/OpenWRT/RouterOS or a PC with IPCop/SmoonthWall.
And as mentioned earlier in the thread, even if you have limiting software installed, it's relatively easy to bypass it, usually by removing it. Or boot into another OS where the software doesn't run, so it doesn't do accounting.
Your only choice if you want to limit is a hardware or software firewall solution.