Workaround:

public Form1()
{
    InitializeComponent();
     
    this.radRibbonBar1.KeyTipShowing+=radRibbonBar1_KeyTipShowing;
}

private void radRibbonBar1_KeyTipShowing(object sender, CancelEventArgs e)
{
    RadButtonElement b = sender as RadButtonElement;
    if (b!=null && b.Enabled==false)
    {
        e.Cancel = true;
    }
}