The issue only reproduces on Windows 10( 1806). Check the attached video showing the result: 

Workaround: Handle the CommandExecuting event this way
private void RadRichTextEditor1_CommandExecuting(object sender, Telerik.WinForms.Documents.RichTextBoxCommands.CommandExecutingEventArgs e)
{
    if (e.Command is ChangeFontFamilyCommand && e.CommandParameter.ToString() == "")
    {
        e.Cancel = true; 
    }
}