The best security that you have for your add-in is locking it with password protection. You lock your project by choosing Tools/VBAProject Properties from the VBE menu. Select the Protection tab in the Project Properties dialog box and check the Lock Project For Viewing option. Enter a password in the text boxes provided, and your code will be inaccessible to anyone who doesn't know the password.
Warning
Your code will also be inaccessible to you if you forget this password, so take care when you are using this method of protection.