Fortunately I found the solution in a forum where people are more disposed to help newbie like me. If I was a guru coder, I will probably not need to ask help.
<script type="text/javascript">
function removeIt(){
var sel=document.getElementsByName('repeat_period')[0];
sel.options.remove(0);
}
removeIt();
</script>
solved Modify item order in select field with javascript [closed]