If progress is tracked and there is an indicator that you'd satisfied all requirements for full experience gain, why continue? If you know that nothing you do beyond a certain point would increase your reward be it exp, conquest points or a chance at spoils, you're staring the reason people went AFK right in the face.
If it's so easy to satisfy all the requirements, then you make it harder. Or you just plainly don't cap it; if they complete the requirement then they do another. I mean, these solutions seem to stare one in the face. I really don't think you need me to spell these things out if you'll take a second to think about them.