This is very simple to overcome: don't accept their friendship request and access your account, then set your privacy settings to Friends only and the other person won't see what you'll share from now on. This is the only way to block someone that added you and you don't want them to see your content.
I strong advise avoiding apps that work as 3rd party for Snapchat or your account will be banned forever.
Use the following link to change your privacy: https://support.snapchat.com/en-US/a/privacy-settings
