The easiest way is to handle the ToolMenuStripItem DropDownOpening event. The
sender
parameter tells you which menu is opening:
private void editToolStripMenuItem_DropDownOpening( object sender, EventArgs e )
{
ToolStripMenuItem tsmi = sender as ToolStripMenuItem;
tsmi.DropDownItems[0].Enabled = false;
}
Would disable the first item of which ever menu is opened.