Because you pay for what you use, what your server's individual user's use will cost you money.
Ultimately you'll have to use the tools available to you in cPanel to restrict things like disk space. With their quotas feature, you can effectively limit them on capacity.
CloudLinux has a number of helpful tools that would allow you to control your individual users. With CloudLinux, you can influence their CPU and Memory limits. You indirectly influence how our hybrid storage responds to a user's data with CloudLinux's IO limits. Using CloudLinux, you can also monitor a user's usage, and in theory you could extend utility billing to them to some extent. You can find CloudLinux (and buy a license) by going to their site.
In the future, we'd like to allow better insight into the individual user's usage, especially as it relates to our hybrid storage platform.