The mapping of a part, which contains the symbols, is read only. To remove symbols from the mapping, you must create a new part and copy all the information to the new part, removing the symbol as the data is copied.
So I cannot delete symbol mapping for existing part record.
I have to recreate this record, am I correct?
You are correct. You must create a temporary part number (I use "_temp"), swap the information over to the new part (modifying what is needed), commit the new temp part, remove the old part, then rename the temp part to the old part.