I’m not familiar with that model off the top of my head but it looks like a standard oven, are you sure that the model number isn’t SE210X/1 as that makes more sense.
If it is then simply turning the main function control knob to the off position (“O”), normally pointing the knob straight up towards the worksurface, should switch it off.
If that is not working then you may well have a problem with the switch behind that and you’ll have to switch it off at the mains.
Hope that helps.
K.