How can I stop this drain pipe from backing up?
The plumber you had out kind of left you hanging.  He should have put a camera down there to give you more details on what is going on.    

Basically your septic tank is backing up or you have an issue with the main line.  It could be slight collapsed or pinched or have roots growing in it or whatever.  

So your first step is to have someone come out with a camera to tell you what the issue is.  For something ongoing though like this I am guessing you have a pretty big job so brace for the worst.  From your symptoms I am going to guess you have an old clay main line that is broken/collapsed.  To replace this it involved a lot of jackhammering, digging and moving concrete.