How do I set a variable so that after a condition is met the loop ignores input?
You've already described what you need to do.  Just set a boolean variable.  It really couldn't be easier. 

boolean runThisPart = true;

void loop() {

  if(runThisPart) {
     // this part only runs if runThisPart is true
     //  if something happens and you don't want
     //  to run this part anymore then
     if(some-condition){
        runThisPart = false;
     }
  }
  //  The rest of your code can also set runThisPart
  //  to true or false to turn that section on or off.
}