I've changed my password and tried using gmails sign off other sessions button with no success (they are still logged in and txting). Is there a way to force the end of that session?
Here is a Google Group conversation where a user had a problem very similar to yours. Apparently, logging off in Google Voice should force the person out.
Also, Gmail has a feature that let's you sign off all sessions:
I'm not sure if you tried those specific steps or not, so they're included anyway.
One thing not mentioned is that you can use another Google Account and get another phone number. As long as you bind that number to your same cellphone number, it will disconnect that cellphone from your other number.
This might stop the text messages. It depends on whether or not Google requires that your Google number be connected to a phone.