If you earth the autofill wire, in this case it would be easiest to just touch it to the boiler, the autofill should stop. Earthing it replicates the probe touching the water in the boiler.
Possibly the autofill probe has a build up of scale which is stopping it from touching the water and telling the autofill to switch off.
Remove the probe and clean it with a wire brush or sandpaper to remove any scale. This should hopefully sort it out.
If this doesn't work or earthing the wire doesn't stop the autofill then there is something else wrong.
Check the wire for any damage or breaks.
If there's still no luck then the autofill on the circuit board is probably faulty. This is quite rare though.