Hart confronted referee Mark Cowburn at the end of Forest's league fixture against Reading following Jack Lester's sending off for two cautions.
A Nottingham Forest spokesman told the club's website: "Paul Hart has today been fined £500 and warned as to his future conduct."
Harty denied using foul and/or abusive language to the FA who took into account his previous record and Forest's improved disciplinary since he took the helm.