Hi Raja,
A solution is: use another cell on the sheet and excel functions to convert the original value before displaying. The dashboard output control can be pointed to this new cell.
Useful excel functions for the conversion are IF and SUBSTITUTE. For example (suppose A1 is your original output cell):
=IF( A1="#", "", A1 )
=SUBSTITUTE( A1, "#", "" )
IF will be better option if you have a text dimension and you expect the "#" character to be present inside the values. For example, suppose value is "Item #12: Product X". SUBSTITUTE would convert incorrectly this value to "Item 12: Product X" because it will work on all occurrences of "#".
Regards,
Fernando