There are two hooks you can use here: frm_setup_new_fields_vars and frm_setup_edit_fields_vars. You can redefine your drop down and filter the results with SQL.
And how do I filter results with SQL
You write a SQL SELECT statement, use the the wpdb object to execute it and apply the results to your dropdown. The SQL SELECT should only pull the values you want. There are plenty of examples to learn from in the knowledge base articles for the two hooks.
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