If this is the case, this indicates that the on/off valve is not closing correctly. This is the middle knob on the front of the valve where you turn the water flow on to take a shower. The most common cause of this can be a build-up of limescale particles which can form within the cartridge, thus not allowing it to fully shut off. In the first instance, isolate the water, and remove the valve. Below is our "How To" video which shows you how to remove the cartridge. Once you remove the cartridge, inspect for any obvious signs which point towards why it may be failing and clean the inside of the cartridge housing (the valve). It's likely given that the valve is a closed unit, you will have to request a replacement (if under warranty) by submitting a ticket along with your warranty number. If you're outside your warranty, you can purchase a replacement here 

NOTE: There are 2 types of on/off valves measuring 2 different sizes. Please ensure you remove the cartridge from your valve and measure it to confirm the exact one you require.