What is the primary means cartels use to control the price of the goods they produce

controlling supply levels

It is commonly understood in economics that competition lowers prices by having companies fight each other for the consumers earned dollars.

A cartel is, then, an organization that exists for the purpose of controlling the supply and production of an item in order to fix the price.

By eliminating competition, a cartel is able to dictate to the market what they believe is an acceptable or desirable price.