We're sorry for your frustration!
Users with our app can send text messages to contacts in their address book inviting them to sign up for Voxer. Unfortunately, we cannot control who sends these invites to their contacts and are therefore unable to block specific numbers from receiving these invites.
Most cellular carriers offer a service to allow you to block incoming messages from certain numbers. You may want to try contacting them for assistance and see what type of solution they can offer. You can also try responding to the number sending you the invites and let them know your concerns.