To Stop the incoming messages from the number, there is a provision, a checkbox called SMS Opt-Out by selecting which the outgoing text messages will be restricted and no messages can be sent to that number.
1. Go to the record of any object lets say lead, and select the Opt-out checkbox and click on save.
2. You can see an alert that will be displayed as below once the SMS Opt-Out option is selected while sending the SMS as shown below.
3. To enable the Send SMS for the number again you need to either uncheck the opt-out checkbox or just click on the button called Request for Opt-In.