The bioethanol stove is controlled by the flap on the bio box. This is opened and closed with the stainless steel hook provided. When the flap is closed, the flames should stop. In some cases, this cannot fully extinguish the flames.
It is important not to panic and to remember that the fire is contained in a box and will eventually die out anyway. In some cases the flap may just need pulling back and forth a few times. The most likely causes of this happening are:
There is some fuel on the underside of the flap. In this case, you need to wipe down the flap and try again.
The flap has become distorted and needs replacing. If distorted, it will continue to let air in underneath, continuing the burn. To check if it is flat, simply place it on an object that is known to be flat and see if it is raised at any point.
Incorrect fuel. If the fuel is wrong, or has a higher than advertised alcohol content, it can be harder to close down. This will often be in conjunction with the flames burning higher and hotter.