Obtaining complete and reliable data on the consumption of server resources (RAM, CPU) for each USER on linux and freebsd it is impossible. You can take a snapshot of the statistics every N minutes (for example from ps), and assume that this reflects the real picture =) In reality, the process can work between charges statistics, to devour all the resources and completed, leaving you a trace. Or for example to eat other resources — iops's hard drive, without giving load on Prots, but shipping the entire system. Or, if we are talking about hosting, making requests to mysql, for which wartime shot.
On the other hand (as is the case in all sorts of cpanel) — just Google with Yandex to visit the site as work limits for instantaneous overload % (depending on settings) can lead to immediate blocking of the hosting.
However, let's not talk about sad =) a Snapshot of the data to remove can and should still nothing else to do. Ready monitoring system in the visualization of these data in you it will hardly help, except maybe Cacti, piles of scripts and large files — the number of users in the system can vary, and even cacti will have to press refresh