Just go to your Itunes Music folder in finder (search with spotlight) and delete "itunes music library.xml" you will have a clean sheet!
If you want to delete songs who misteriously won't delete the regular way, but you do not want to delete your entire library there is a solution too!
1. Search for Itunes in Spotlight after you have closed the app.
2. Open The iTunes folder.
3. Open the Itunes Music Library.xml by command click (or right click) and choose "open with" then choose "texteditor.app".
4. Now you wil see alot of symbols and letters, don't panic!
5. Hit Command+F (Search function) and Search for one tipical word in the title or artistname of the song you want to delete. (like in Daft Punk you choose to look for "Daft")
6. Now it wil give a result within the tekst, try to read in between all the symbols, the artist and trackname are there (it's a collection of all the info from the ID3Tag of the song)
7. If it is the song you want to get rid off delete all the tekst in between both:
</dict>
<key>....</key>
<dict>
8. Example:
</dict>
<key>1522</key>
<dict>
<key>Track ID</key><integer>1522</integer>
<key>Name</key><string>Vondelpark Vannacht</string>
<key>Artist</key><string>Acda & De Munnik</string>
<key>Album</key><string>Acda & De Munnik</string>
<key>Genre</key><string>Nederlandstalig</string>
<key>Kind</key><string>MPEG-audiobestand</string>
<key>Size</key><integer>3223032</integer>
<key>Total Time</key><integer>200594</integer>
<key>Track Number</key><integer>4</integer>
<key>Year</key><integer>1997</integer>
<key>Date Modified</key><date>2009-05-06T22:50:44Z</date>
<key>Date Added</key><date>2009-03-22T13:52:36Z</date>
<key>Bit Rate</key><integer>128</integer>
<key>Sample Rate</key><integer>44100</integer>
<key>Artwork Count</key><integer>1</integer>
<key>Persistent ID</key><string>85F4AC28025C382E</string>
<key>Track Type</key><string>File</string>
<key>Location</key><string>file://localhost/Volumes/TERA/Media%20Files/Muziek/A/Acda%20&%20De%20Munnik/Acda%20&%20De%20Munnik%20-%20Acda%20&%20De%20Munnik/Acda%20&%20De%20Munnik%20-%2004%20-%20Vondelpark%20Vannacht.mp3</string>
<key>File Folder Count</key><integer>-1</integer>
<key>Library Folder Count</key><integer>-1</integer>
</dict>
<key>1524</key>
<dict>
You delete the fat printed text and make sure you'll keep the buttom </dict>...etc
9. Now you have deleted the song from the library. Once you start itunes again, it will be gone from your list!!
succes!