prevent from pressing digits without alphabets and spaces

HTML

<input name="username" id="username" type="text" onCopy="return false" 
    onDrag="return false" onDrop="return false" onPaste="return false" 
    autocomplete=off />

JavaScript

$(document).ready(function() {
    $("#username").keypress(function(event) {
        var inputValue = event.which;
        if(!((inputValue >= 65 && inputValue <=  90) ||  // A-Z
             (inputValue >= 97 && inputValue <= 122) ||  // a-z
             (inputValue == 32))) {                      // space
            event.preventDefault(); 
        }
    });
});