The "IRS 1099 Code" field on a Purchase Invoice is usually grayed out if the invoice is posted. However, it seems that for your case, the field is no longer grayed out and you can remove the 1099 code type now. This can sometimes happen due to a temporary system state or a specific process completion.
For already posted invoices, if the field remains locked, the standard method to adjust 1099 taxable payments is to directly edit the "1099 Amount" on the Vendor Ledger Entry.
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