Dimeritium is a special sort of metal which suppresses magical power. Vilgefortz of Roggeveen, Artaud Terranova and a few other Chapter members, were shackled with dimeritium after they were accused of treason by Philippa Eilhart and Dijkstra during the reunion of mages on Thanedd Island. It was stated that being shackled causes unpleasant side-effects for the mages.
"Dimeritium is a rare and precious metal with an interesting feature — it represses the transfer of magical energy. A bluish alloy of iron and dimeritium is used to produce handcuffs and necklaces. Those who wear them cannot cast spells or use magic in any way. There are known cases, however, of extraordinarily powerful sorcerers managing to overcome the shield generated by dimeritium."
Dimeritum plays a larger role in the Witcher 3, being used to craft weapons and armor, it's difficult to find but not impossible. One source of dimeritium are witch hunters, during some missions where you kill them you can loot their bodies and find dimeritium shackles. Due to the widespread persecution of mages during the game, dimeritium is used extensively by people to successfully subdue mages and render them helpless. Lastly, the metal can also be used to craft bombs that cancel out monsters abilities, be warned however, dimeritium also has the ability to cancel out Geralt's signs if you're within the blast radius, leaving both opponents vulnerable.