Having an anti-harassment policy and making sure your employee have had training in what harassment is and what is considered unacceptable behaviour are good foundations for your zero tolerance stance towards harassment. But you need to make sure that your policy, and the training, are brought into every day life in your business. If you have a small business, you will need to take steps to ensure that your employees know that the zero tolerance stance is real and you are serious that it is followed. If your business is bigger, your line managers will have a responsibility here.
First, make sure your training method is appropriate. Online training can be effective but you should check that employees are actually watching it, rather than just saying they have. This can help those employees fully understand what is classed as harassment and therefore which actions may mean they are breaching your policy.
Then you, or your line managers, need to set an example to their team. They should call out behaviour that risks crossing the line, and empower their employees to speak up if they have any concerns about behaviour towards them or other people. You should consider whether it's appropriate for you or your managers to be in social media chats, for example Whatsapp, with their staff as lapses in judgment can occur.