CPanel has added mod_bandwidth into the easy apache so you can just run this to limit bandwidth on an account:
/scripts/setbwlimit --domain=your-main-domain.com --limit=8000000
I am leaving the old post just for reference below:
keep in mind mod_bandwidth does not work with Apache 2.0, only Apache 1.3. The best option for Apache 2.0 is called Mod_Cband (Click the link for more details)
Mod_bandwidth has changed to adding the varibles in one line code, now you must add the domain name and the limit all in one shot like this:
/scripts/setbwlimit –domain=example.com –limit=8000000
Here are some KB to MB translations.
8000000 – 8 mps
5000000 – 5 mps
3000000 – 3 mps