How do I ensure that a search committee won't contact my current colleagues (that are not references) to ask about me?

I used the following language, and I think it had the desired effect.

Because I am hoping to keep this search confidential, I request that you contact my references rather than my current colleagues at this time. If I were to become a finalist, I would of course expect and invite you to speak to my current department.

I have seen cover letters like that, here honesty is key. Remember that people on the committee would like to have the best candidate get the job, and that sometimes requires some "poaching".

To paraphrase a good sentence I have previously seen used to that effect: "I am currently employed at institution X. I am happy with my employment here, but ready to seek new challenges at institution Y. As my employment at institution X is still ongoing, I would appreciate your discretion when inquiring references. Should you need references from my current colleagues at X, please contact me in advance."

I don't think there is a way to guarantee it, but I also think the practice is pretty rare. It might occur when someone at the new place knows someone where you currently are, of course.

You also can't expect them to never ask but can request that any contact not come immediately because you don't want to prejudice your current administration against you thinking you are about to leave.

I suspect that it is pretty common to be in your situation, so people receiving the request wouldn't see it as unusual.

If people have the impression that you are happy where you are but exploring possibilities elsewhere and that you'd like things to stay confidential for a while, I think most people would accept that and rely initially on the materials you send them. Later in the process they might want to talk to your current boss, of course.

But you can't actually guarantee that it will stay quiet.