Twilio - how to disable inbound SMS
The install instructions at https://wiki.civicrm.org/confluence/pages/viewpage.action?pageId=86213770#SettingupaSMSProviderforCiviSMS-twilio say

In Twilio account settings, go to Numbers and click on the number you want CiviCRM to "listen to". That will open configuration settings for that phone number. Under Messaging, change Request URL to http://yoursiteurl/civicrm/sms/callback?provider=org.civicrm.sms.twilio with HTTP POST

So if you remove that url it should stop posting inbound to CiviCRM.