Thumpers
Thumpers are the kind of anti-psionic devices you get when psionic abilities start showing up in the population at an early-medieval level of technology. They’re crude, brute-force, and indiscriminate in their effect — but they work, which is the important thing. In fact, to the people in power that typically commission the devices, that’s really the only thing.
At its heart, a Thumper is simply a mechanism where a large piece of quartz (although diamond works, too) is regularly smacked in a particular way by a hammer hard enough to generate an electric charge. Not that the people who use them know anything about electricity: they just know that anybody with psionic powers who is too close to a Thumper when it’s operating gets horrible headaches, and loses fine control of his powers. The only way to avoid the pain is to ruthlessly damp down one own’s powers, which makes it rather easier to confine someone with psionic abilities. It’s also an excellent way to detect potential psions; any kind of active user is going to be vulnerable to a Thumper.
The major restriction of Thumpers is that they’re not portable. They are, in fact, generally hooked up to waterwheels, on the principle that machinery can’t be mentally dominated by a sufficiently-motivated telepath. That this leads to Thumpers being put places out of the way — thus making them more susceptible to daring rescue raids by adventurers — is an ongoing problem, at least for rulers. Also: generally speaking, a Thumper can be maintained and repaired by any competent blacksmith or stonemason. But, if left unsupervised, it starts to work erratically after a month, and usually breaks down completely after three. This can likewise lead to adventures, if not the sort of adventures that adventurers actually like.