Are you using Windows? Do you mean that you see a shared 'your name's Limewire tunes' within itunes? If that's the case, it's just a playlist...the files haven't actually been added to the iTunes library. So, to delete files from that playlist, delete from within LW's library or from within your saved folder.

Make sure that you really want the songs deleted...if you delete from LW's library & don't choose the option to send files to the recycle bin - they're gone & you'll need a file recovery program to even attempt get them back!!

If you've actually added the files to iTunes library, just right click > delete.

It's probably worth you reading this thread, there's a lot of info & tips for LW & iTunes
Transferring music to iPod & iTunes

You can't preview a song before download, you can only preview during the download.