The following function removes all the non-digit numbers from a text field (so it also works with pasting a piece of text):

  function numbersOnly(field) {
   field.value = field.value.replace(/[^0-9]/g, "");
  }

Use this function in combination with the following HTML snippet example:

 <input type="text" size="8" onkeyup="numbersOnly(this);"/>