Before you start deleting stuff, there is one GIANT thing you need to watch for. Make sure you only have ONE song per folder. If not, following the steps these guys described above (which is the correct way) will delete other songs also.
For example.... You have a folder for your music, then you start a new project in it called song 1. You record a bunch of stuff and everything is great. The you do a new song called song 2 and you put it in the same folder as song 1. You do tons of tracks on song 2 and then decide to get rid of a bunch of them. You delete them from your arrangement window and everything is good. Now you go into the Audio Pool and select remove unused media. On the pop up you select erase from hard drive. This is kind of the standard way of going about all this to free up space. What you may not have noticed is that because songs 1 and 2 were in the same folder, the Audio pool referenced the files from both projects. When you had song 2 open and selected remove unused media, all the tracks from song 1 that were not part of song 2 got treated as unused media. When you erased the files, song 1's audio files just went with it.
Another thing to watch for is templates. When you make a template file, it stores routings, channel counts etc... If you already have audio or data in the project when a template was saved, it saves witht the template.
For example. You take song 1 from before (now assuming that songs 1 and 2 are properly saved in seperate folders) and you save a template after doing song 1. Now when you start song 3, you start with your template from song 1 because you like the way it is laid out. When Song 3 loads you will notice that all the tracks from song 1 loaded also. So now you delete those files form song 3 so you can have a fresh start. At the end of song 3, you do your whole remove unused media thing, now song 1 will be gone. With the files in the template, the audio pool viewed them as unused in song 3 since you deleted them from the arrangement, but they happened to be the files needed from song 1.
To make a long story short, in the whole remove unused media process, there are some options. One of those options is to remove something from the pool, and not actually delete it. With shared files, removing things from the pool is better because it leaves the file for whatever else may need it.