In Home tab, at Number section, in the format drop down list, the last line is More number formats. Click on it, it will open the format window. On the Number tab, on the left side, the last entry in the list is Custom. Click on it, then in the right side you will see a list of custom format. You will find in that list the wrong formats that must be deleted... select the entry , and Under that list there is a Delete button.