rad fan continuing to run after turn off?

i have a vauxhall vectra p reg .my problem is that when the engine is switched off the fan continues to run.i disconnected the plug to switch which is screwed on the radiator,but it still continues to run.i did not touch the thermostat as i am not sure if this could be the problem or some other.please advice possible causes to this problem.

thanking you in anticipation
g.best