On physical web servers running CentOS 5 (x64) or RHEL 5 (x64), disk quotas do not work because of a problem in the Linux kernel (see https://lkml.org/lkml/2007/6/18/59 for details).
The recommended solutions are
1. Use Parallels Virtuozzo Containers (PVC) VPSs to deploy web servers for the CentOS 5 (x64) or RHEL 5 (x64).
2. Use Linux Shared Hosting NG service.