Restrict multiple category selection

I have a multiple select option in a form, so the user can choose more than 1 category by holding down control and selecting. This means they can select any number of categories.

Is there any way to restrict this so they can only select say 1 option? Is this possible?
dartagnonAsked:
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David H.H.LeeCommented:
Hi dartagnon,
I'm curious why you not use normal option selection box for this selection instead of listbox? But, here is one of the attempt that restricted only one selection in listbox as requested.
<select id="sel1" multiple="true" onclick="changeOpt(this);">
<option value=1>1</option>
<option value=2>2</option>
<option value=3>3</option>
</select>
 
<script>
function changeOpt(sel){
  var opts='';
  var intSel=0;
 
  for(i=0;i<sel.length;i++){
    if(sel.options[i].selected){
      intSel+=1;
     
     if(intSel>1){
      sel.options[i].selected=false;
     }
    }
  }
}
 
</script>

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Michel PlungjanIT ExpertCommented:
If you want to SHOW the options but still only have one selection, just change the size

<select size="4">
<option value="">Please select</option>
 <option value="1">One</option>
 <option value="2">Two</option>
 <option value="3">Three</option>
</select>
 
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dartagnonAuthor Commented:
Thanks x com, that worked brilliantly. Here is what I ended up with:
<select name="categoryID" size="6" multiple="true" onclick="changeOpt(this);">
                            <option value="0" selected="selected">-- Select Category --</option>
                            <%
While (NOT categories.EOF)
%>
                            <option value="<%=(categories.Fields.Item("categoryID").Value)%>" ><%=(categories.Fields.Item("categoryname").Value)%></option>
                            <%
  categories.MoveNext()
Wend
If (categories.CursorType > 0) Then
  categories.MoveFirst
Else
  categories.Requery
End If
%>
                          </select>
                                      <script>
function changeOpt(sel){
  var opts='';
  var intSel=0;
 
  for(i=0;i<sel.length;i++){
    if(sel.options[i].selected){
      intSel+=1;
     
     if(intSel>3){
      sel.options[i].selected=false;
     }
    }
  }
}
 
</script>
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