What's the noun for monopolises? Here's the word you're looking for.
monopoly
A situation, by legal privilege or other agreement, in which solely one party (company, cartel etc.) exclusively provides a particular product or service, dominating that market and generally exerting powerful control over it.
An exclusive control over the trade or production of a commodity or service through exclusive possession.
The privilege granting the exclusive right to exert such control.
“A current issue in many countries is the monopolisation of media by powerful interests, whether private or public.”
“This provision will prevent the monopolisation of whole markets, which could have happened in the absence of such a provision.”
“A current issue in many countries is the monopolisation of media by powerful interests, whether private of public, which lessens the plurality of voices in the public sphere.”