You can handle the CurrentCellStartEditing event and cancel it if the current cell is a formula cell.
Private Sub GridControl1_CurrentCellStartEditing(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles GridControl1.CurrentCellStartEditing
Dim cc As GridCurrentCell = Me.GridControl1.CurrentCell
If cc.Renderer.GetType() Is GetType(GridFormulaCellRenderer) Then
e.Cancel = True
End If
End Sub