JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="id_del_campo"/>

JavaScript

var input = document.getElementById('id_del_campo');


  input.addEventListener('textInput', function(e) {

      //Capturamos los datos
      const caracter = e.data;


      const caracterKey = caracter.charCodeAt(0);//Ehemplo  a = 97

      if (caracterKey === 97) {
          console.log("Se escribio la A")
          //e.preventDefault();
          return false;
      }
      return true;
  });