As a system builder I don't want my clients overclocking their systems. How can I find motherboards that don't allow overclocking? Is it possible to disable overclocking on existing motherboards (via the BIOS).
Typically, if you are purchasing off the shelf components, you are not able to do this.
You may want to ask the manufacturer if they can provide you with a version that has this disabled.
Apart from that, I know Intel for their boards have the Intel Integrator Toolkit which does what you want - so you may just want to join the partner programs of companies you use on the off chance they have similar tools.
That being said, there isn't anything stopping them from reflashing. The only 100% certain thing you can do is to buy parts that do not allow it - e.g. Intel H67 instead of P67 boards.