If an OP wants to, they can set mode +m in the channel. This means that the channel is moderated. This allows the operator to voice (+v) someone so they can talk. Anyone without a voice is not allowed to talk in the channel.

Images:

With no voice:


With voice:


Additional note:
If the channel is not moderated, the voice option is frequently used as a status symbol. When there is no +m, voices have no more privileges than the regular users.