I believe that this message is based upon the market value of the corporation in question. Not 100% sure... but you may want to try tanking the market value (set 10% hiring or 10% production to cripple profitability) and try closing the corporation after it has fallen. Not sure what the point was where this message no longer prevents you from closing, maybe 200B or 300B.
This is mostly speculation. I haven't encountered this in a long time.