Refrence the [guide|1029|Starbucks Barista teardown] to get inside of the machine. At step 8, loosen the 2mm hex screw on the steam valve knob and you should be able tighten/close the steam knob more than before. Re-tighten the 2mm hex screw, reassemble the machine and test it. If this doesn't solve the problem you may have some calcium blocking the valve from closing all the way, which a descale should take care of.