I've had this on a 2930 & found that it was a chunk of limescale stopping the magnetic valve from closing. I removed the valve piston & cleaned out the limescale & restored functionality.
The piston is screwed into the tube that the solenoid is attached to. Remove the solenoid & gently use grippers to unscrew the valve watching out for the spring etc.
Have a good look inside the tube & clean as best you can.
Reassemble carefully & you should only have steam when pressing the iron button.
Another way may be to fill the heater, where flushing plug is, with 50/50 water & vinegar & switch the express on. This mix should disolve the limescale blocking the valve & stop the steam. Might take a while, try the mix in a kettle & see what it does!
Keep this site informed of progress as others will benefit.
The Fixer
January 2007