Unfortunately, one-way messaging doesn't really exist.

It's a limitation of the actual telecom networks (AT&T, Verizon, T-Mobile, etc). There's no way for a phone number to send a message but not be able to receive messages in return.

Our recommendation is to simply inform your contacts that responses aren't monitored in your first message. Some of our users put a message that says "DO NOT REPLY" at the beginning or end of their message.