Maybe there will be a rule that if someone is so wireheaded they are unable to answer a simple question within sufficient time limit, euthanasia is legal, and their hardware is given to the remaining ems. That could be a selection pressure against wireheading.

This could be solved by splitting oneself into two personalities, the larger one would get the pleasure, and the smaller one would be responsible to prevent euthanasia. But when the larger personality stops responding, the smaller one could kill them and take their resources. Or the smaller one could try to split to two parts...

Okay, at the end either there is a part that avoided wireheading, so at least that part participates in our reality, or no such part remains and then the whole system can be legally killed.

Another way out would be if 1% of people resisted wireheading for whatever reasons (religious, or program themselves to be unable to accept wireheading), and gradually would take over the world.