disabling incoming SMS

I'm seeing a lot of this in my logs:

2019-02-18T19:28:26.526495-08:00 vishnu smsbox[21580]: 2019-02-18 19:28:26 [21580] [5] INFO: Starting to service <#001#006'application/vnd.wap.mms-message> from <1111301000> to <1234>
2019-02-18T19:28:44.167763-08:00 vishnu smsbox[21580]: 2019-02-18 19:28:44 [21580] [5] INFO: Starting to service <11111301000 Error Invalid Number. Please re-send using a valid 10 digit mobile number or valid short code. No service specified> from <+1121611611> to <1234>
[...]
2019-02-19T08:38:24.918834-08:00 vishnu smsbox[21580]: 2019-02-19 08:38:24 [21580] [5] INFO: Starting to service <11121611611 Error Invalid Number. Please re-send using a valid 10 digit mobile number or valid short code. No service specified> from <+1121611611> to <1234>
2019-02-19T08:38:37.474384-08:00 vishnu smsbox[21580]: 2019-02-19 08:38:37 [21580] [5] INFO: Starting to service <11121611611 Error Invalid Number. Please re-send using a valid 10 digit mobile number or valid short code. No service specified> from <+1121611611> to <1234>

I speculate:  someone has found my cellular modem number and is sending me spam.

For this application, I transmit SMS over the cellular modem; I don't want to receive anything.

  *   My carrier (AT&T) tells me that they cannot block in-bound SMS - they can enable bi-directional SMS or disable it:  all or nothing.
  *   I disabled auto-answer on the modem ... but this setting appears to govern POTS, not SMS


  *   Is there a Kannel way (bearerbox or smsbox) to disable incoming reception of SMS?
  *   Alternatively, is there a way to instruct Kannel to send requests for Invalid Numbers (e.g. '1234') to /dev/null?
  *   Other ideas for dumping these messages?  I'm mostly annoyed by the repetitive logging

At the moment, I have a cronjob job which stops bearerox & smsbox, sleeps for several minutes (not clear to me why this is necessary), and then restarts them.  This of course creates a dead zone, in terms of service, which is suboptimal.

--sk