combo box to ignore list

I have a userform with a combo box on it that shows the worksheets in the workbook. What is selected is then saved to sheet 'A' column 'A'. I have a NameRange that is not dynamic called 'used_sheets' that lists all the sheets that have been selected. How can I get that userform Combobox list to ignore the sheets already listed in column 'A'
Such when I select sheet 'H' and it's listed in column 'A', the next time I use that combobox it will not list sheet "H".
The code for the combobox is

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Attached is a sanitized workbook

tried it and it WORKS just as I wanted
THANKS