Hello,
It's hard to say what could be the issue without having the logs and the configuration.
The CLI commands to disable PPTP would be
uci set network.[instance_name].disabled='1'
uci commit network
/etc/init.d/pptpd restart
And if you were to use the SMS UCI interface, the message to send would be:
[admin_password] uci set network.[instance_name].disabled='1'
The rest of the commands should be handled by the router, but in case the instance does not go down, you may need to restart the router.
If you have the router on hand, please replicate the issue (send the commands that did not work), then navigate to System → Administration → Troubleshoot and generate a troubleshoot file. It can be sent to me via a private message.
Best regards,
DaumantasG