<input type="text" placeholder="you can input hiragana only">

*,
*:before,
*:after {
  box-sizing: border-box;
}

body {
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100vh;
}

input {
  padding: 0.5rem;
  width: 20rem;
}

let input = document.querySelector("input");

// disable event function
const disableEvent = e => {
  e.preventDefault();
  e.stopPropagation();
};

// disable paste
input.addEventListener("paste", disableEvent);

// disable drag&drop
input.addEventListener("drop", disableEvent);

// remove string
input.addEventListener("keyup", function(e) {
  let tmp = [];

  this.value.split("").forEach(function(item, i) {    
    if (item.match(/^[\u3040-\u309f]+$/)) {
      tmp.push(item);
    }
  });

  if (tmp.length > 0) {
    this.value = tmp.join("");
  } else {
    this.value = "";
  }
});