Joi custom validation

I'm using Joi to validate some data from user. I'm using it with module like this:

const Joi = require('joi')

// User validation rules
module.exports = {
  create: {
    body: {
      email: Joi.string().email().required(),
      password: Joi.string().min(6).max(128).required(),
      name: Joi.string().max(128).required()
    }
  },
  update: {
    body: {
      password: Joi.string().min(6).max(128).required(),
      name: Joi.string().max(128).required()
    }
  }
}

and then in router I'm passing it as middleware:

router.post('/register', validator(create), userController.register)

For update I want to create custom validator with code like this:

const json = { 
  "email":"aa@aa.pl",
  "password":"someNewPass",
  "name":"John Smith",
  "activationKey":"123123",
  "active":false,
  "resetPasswordKey":"123123"
}

const forbiddenFields = ["email", "activationKey", "active", "resetPasswordKey"];

const validate = (json, forbidden) => {
  for(let i = 0; i < forbidden.length; i++) {
    if(json.hasOwnProperty(forbidden[i])) {
      return false;
    }
  }
  return true;
}

const isValid = validate(json, forbiddenFields)
console.log('is json valid? ', isValid)

I found extend method in Joi API but example is so confusing that I can not handle to create custom validation with my code.