Any suggestions on how I do this?
If you want to disable forum email completely, then look in cron.php and you should be able to determine how to remove the code from there...that (the cron script) is what generates those emails. You could disable cron all-together, but that may stop some things you do want to happen.
The simplest way to prevent students from subscribing to forums is to edit each forum and change the setting "Force everyone to be subscribed?" to "Subscriptions not allowed".
I have different problem, my forum don't send mail to subscriber but can send mail for confirmed register.
What setting that should be done?