self.SelecIG["menu"].config('Ingreso A',state=DISABLED)

self.IG=['Seleccione una Opción',
        'Ingreso A',
        'Egreso A',
        'Ingreso B',
        'Egreso B']

self.SelecIG=ttk.OptionMenu(self.BloqueD1,self.ValorIG,"Seleccione una
    Opción",self.IG[0],self.IG[1],self.IG[2],self.IG[3],self.IG[4])
self.SelecIG.place(x=25,y=25)
self.SelecIG.config(width=21)
self.SelecIG["menu"].config(bg="#3C4966",fg="white")
self.SelecIG["menu"].config('Ingreso A',state=DISABLED)

self.SelecIG["menu"].config('Ingreso A',state=DISABLED)

self.SelecIG['menu'].entryconfigure('Ingreso A', state=DISABLED)

import tkinter as tk
import tkinter.ttk as ttk



class MainApp(tk.Frame):
    def __init__(self, parent, *args, **kwargs):
        super().__init__(parent, *args, **kwargs)
        self.parent = parent
        self.BloqueD1 = tk.Frame(self)

        self.ValorIG = tk.StringVar(self)
        self.ValorIG.set('Egreso A')

        self.IG=['Ingreso A',
                 'Egreso A',
                 'Ingreso B',
                 'Egreso B'
                 ]

        self.SelecIG = ttk.OptionMenu(self.BloqueD1, self.ValorIG,
                                      "Seleccione una Opción", *self.IG
                                      )

        self.BloqueD1.pack()
        self.SelecIG.pack()
        self.SelecIG.config(width=21)
        self.SelecIG["menu"].config(bg="#3C4966", fg="white")
        self.SelecIG['menu'].entryconfigure('Ingreso A', state=tk.DISABLED)


if __name__ == "__main__":
    root = tk.Tk()
    MainApp(root).pack(side="top", fill="both", expand=True)
    root.mainloop()