What sort of alerts are you talking about?
There isn't really code that would directly allow that to happen, though for thread/forum watch notifications, there is time limiting that only sends a certain number of notifications when the reply/new thread is posted, and the remaining ones are triggered in the background as part of page views. This should rarely be an issue though, as a good number of alerts/emails should normally be sent out and there aren't that many that need to be sent for an average thread.