Delete key does not delete the underlying html

I have used freetextbox in my asp.net application. My web page has one freetext box, On command button. On clicking on the command button it insert some text into freetext box. Actually these text are a string of html like this.
<SPAN id=?15aac2e0-0de8-4a39-8653-a5f0e60ad5bd?>Y</SPAN>
in design mode it shows  Y only and in html it show whole html string given above. My problem is when i delete the Y in design mode by pressing delete key from my key board. It only delete Y deos not delete <SPAN id=?15aac2e0-0de8-4a39-8653-a5f0e60ad5bd?></SPAN> tag.
My code is given below.
HTML Code
<FTB:FreeTextBox id="FreeTextBox1"
toolbarlayout="ParagraphMenu,FontFacesMenu,FontSizesMenu,FontForeColorsMenu,FontForeColorPicker,FontBackColorsMenu,
FontBackColorPicker|Bold,Italic,Underline,Strikethrough,Superscript,Subscript,RemoveFormat|JustifyLeft,
JustifyRight,JustifyCenter,JustifyFull;Bul ...