Ahmed,
Please try adding this script to your numeric question:
<script>
$(document).on('keydown', '#[% QuestionName() %]', function(event){
var key = event.which;
if (key < 48 || key > 57) {
return false;
}
})
</script>
Note that this solution currently prohibits decimals as well as copy-and-pasted responses. If either of those are issues, we can look into possible solutions.