Note that you should use the correct DOM id to refer via getElementById.
You can use the .replace() method for that:
var loan_amt = document.getElementById('loan_amt');
loan_amt.value = loan_amt.value.replace(/[^0-9]/g, '');
But that will remove float point delimiter too. This is an answer to your question, but not a solution for your problem. To parse the user input as a number, you can use parseFloat() – I think that it will be more appropriate.