If you really want to get rid of symbol XYZ, try entering a
dividend for XYZ into the name of the investment; type those three
letters, and then hit Tab. Choose Dividend. Note that the name in
the box may have changed. Finish creating the dividend.
Right-click that dividend transaction, and choose GoToInvestment. Click
Details. Whatever that investment is, if the symbol is XYZ, blank
or change its symbol. If the name is XYZ, Rename it to something
else.
The symbol XYZ should now be freed up. Delete the fake dividend.