disable dropdown when once approve/reject

Question

when admin approve/reject any document once then when admin again login then he/she not be able to approve/reject documents again and dropdownlist will be disabled only for those documents which can be once approve/reject then when admin view any new documents then drop down will be enable and when admin approve/reject this document then it will be disable dropdownlist for not approve /reject again
for this i do this
code:
protected void OnRowDataBound
(object sender, GridViewRowEventArgs e
)
 {
 if (e.Row.RowType == DataControlRowType.DataRow
)
 {
//Find the DropDownList in the Row
 DropDownList abc = (e.Row.FindControl("DropDownList9") as DropDownList);
abc.Enabled = false; } }

but this code show me all dropdownlist are disable .
any solution how i will do this?