On its mission to explore new worlds and seek out new civilisations, the spaceship 'Exciting Undertaking' came across a planet of seemingly unadvanced aliens. Following their First Direction (forbidding interactions with pre-wrap-capable aliens and totally not modelled after anyone else's rules of interspecies interaction), they spied on them from orbit and then left.
Little did they know that the species below were actually a race of highly advanced space wizards (people with enough technology that they may as well have been magical) who had abandoned any technology that couldn't be understood with a rudimentary understanding of mechanics (for undisclosed reasons totally not related to accidentally ending the universe once or twice).
The question is how the space wizards (who were clearly intelligent and curious enough to have become space wizards in the first place) managed to suppress their populace from trying to advance beyond renaissance levels of technology over immense timescales (the kind of timescales where facts become ancient history, ancient history becomes myth, and myths fade into obscurity).
Bonus points if your method of suppression also gives the space wizards a way to migrate to another planet when their current home nears it's natural end; without them becoming curious about how it was achieved.
Edit to constrain the questions more: The space wizards were a Kardashev type III civilisation with high levels of control and information processing, leading to them having the ability to cause any physical phenomenon that isn't physically impossible. While very powerful they can't necessarily guarantee that their technology is infallible, and long term systems could break down if not suitably maintained.
Now they have no technology that can't be understood (or at least copied) by looking at it and studying it with macroscopic tools, so no microprocessors, complex balanced chemical reactions or electrical devices. If it requires more than a few minutes of explanation it's a no-go. Forging and simple mechanical devices such as bellows, water wheels and gearing are OK, but further scientific and technological endeavour must be discouraged to reduce the chance of
totally awesome potentially devastating repercussions.
If you're wondering about methods by which the answers to this question will be weighed against each other:
Likelihood of failure: How likely the space wizards are to throw off the shackles and go forth to once again almost blow up everything.
Longevity: How long this suppression is likely to remain in place before suffering catastrophic system failure.
Ease of implementation: How much effort the original space wizards would have to put in in order to put this system in place.
Rule of Coolness: More of a jokey (hugely subjective) category, but we all know that cool things work better in space.