RHEL stop x-server

Very simple. According to the generic runlevel specification, you just have to change to runlevel 3. (Typical systems start in runlevel 5 because of the GUI)

Typical Linux runlevels

ID	Name	Description
0	Halt	Shuts down the system.
1	Single-User Mode	Mode for administrative tasks.[2][3]
2	Multi-User Mode	Does not configure network interfaces and does not export networks services.[4]
3	Multi-User Mode with Networking	Starts the system normally.[5]
4	Not used/User-definable	For special purposes.
5	Start the system normally with appropriate display manager. ( with GUI )	As runlevel 3 + display manager.
6	Reboot	Reboots the system.

How to do it?

# init 3  (as root. And that's it!)

Curiosity

Lately, many systems have migrated to Upstart which is "an event-based replacement for the traditional init daemon — the method by which several Unix-like computer operating systems perform tasks when the computer is started. It was written by Scott James Remnant, a former employee ofCanonical Ltd." (source: Wikipedia)

It was firstly introduced in Ubuntu 6, and was recently adopted by RedHat to its Enterprise Linux (RHEL) as well, being therefore available in dependent distributions like Scientific Linux and Centos.