using DevExpress.XtraBars.Docking;
// ...
private void Form1_Load(object sender, System.EventArgs e) {
   dockManager1.DockingOptions.ShowMaximizeButton = false;
}

private void dockManager1_StartSizing(object sender, StartSizingEventArgs e) {
   // Get the container panel that owns the panel being dragged.
   DockPanel parentPanel = e.Panel.ParentPanel;
   if(parentPanel != null) {
      // Disable the sizing operation when the panel's bottom edge is being dragged.
      e.Cancel = e.SizingSide == SizingSide.Bottom;            
   }
}

Imports DevExpress.XtraBars.Docking
' ...
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) _
Handles MyBase.Load
   DockManager1.DockingOptions.ShowMaximizeButton = False
End Sub

Private Sub DockManager1_StartSizing(ByVal sender As Object, ByVal e As StartSizingEventArgs) _
Handles DockManager1.StartSizing
   ' Get the container panel that owns the panel being dragged.
   Dim parentPanel As DockPanel = e.Panel.ParentPanel
   If Not parentPanel Is Nothing Then
      ' Disable the sizing operation when the panel's bottom edge is being dragged.
      e.Cancel = e.SizingSide = SizingSide.Bottom
   End If
End Sub