More about e-mail subscriptions
If students do not wish to receive an email every time someone posts to a forum in Moodle, they can turn this off by unsubscribing from a forum, but only if the teacher has turned on this option in the forum settings. The teacher can do so in the settings of the forum by making sure that "Force everyone to be subscribed?" is set to "No."
Your students then proceed as follows: First, log into Moodle and open the course that contains the forum. Then, click on the name of the forum. On the upper right hand side of the page, click the link "Unsubscribe to this forum."