Note: ensure all the bolts and fittings are removed as in c5martins first post BEFORE trying this ! This is in addition to Martins advice and assumes you have already completed those steps..
Some times the "hold and twist" method will not work, if the handle bar stem is seized then you may end up damaging the steering rod plate at the bottom.
When more brute force is required, I find this method works;
I use two socket set extension bars as a "drift" to hammer out the handle bar stem from underneath.
It's easier to do this if you place the c5 upside down - a "Black & Decker" workmate is ideal .
Having tried other methods with varying levels of success I find this way has worked without causing any chassis damage or any other snags on four handlebar stems so far - including a couple that were really rusted in. It's my preferred method for now until I discover an easier way