Option Explicit
Private Declare Function CreateRectRgn Lib "gdi32.dll" (ByVal X1 As Long, ByVal Y1 As Long, ByVal X2 As Long, ByVal Y2 As Long) As Long
Private Declare Function SetWindowRgn Lib "user32.dll" (ByVal hwnd As Long, ByVal hRgn As Long, ByVal bRedraw As Boolean) As Long
Private Declare Function GetClientRect Lib "user32.dll" (ByVal hwnd As Long, ByRef lpRect As RECT) As Long
Private Type RECT
    Left As Long
    Top As Long
    Right As Long
    Bottom As Long
End Type
 
Private Sub Form_Load()
    DeleteCmdBorder Command1.hwnd
End Sub
 
 
Private Sub DeleteCmdBorder(ByVal hwnd As Long)
    Dim Rec As RECT
    Dim hRgn As Long
    GetClientRect hwnd, Rec
    hRgn = CreateRectRgn(3, 3, Rec.Right - 3, Rec.Bottom - 3)
    SetWindowRgn hwnd, hRgn, True
End Sub