The one that usually "floods" you is when you watch a whole forum - the result is that you get an alert and email everytime someone starts a new thread or replies!!
These can be turned off by clicking the
Watched Forums link in the navigation bar, ticking the boxes for the forums you want to unwatch, and then (at the bottom) select Stop Watching Forums and click the go button.
Alternatively, when you click on a forum and get a list of threads to view - there's an Unwatch Forum link at the top right of the list.
The other alerts you might get are for replies to your posts, but these are generally setup without email by default.
Hope this helps.
Cheers,
Shaun