We are receiving texts from a random number. What is happening? How can we stop it?

Your texting numbers are just mobile phone numbers, and anyone can text in, just like your number. If you see a strange text, it's likely spam, a mistyped number, or something else.  

How can we stop it?
You can deactivate the user to prevent future messages (do this under Contacts).

Click on the little profile icon next to the participant to go to their Contacts Profile:

Click on the Deactivate button: