Garbage can
 

What does the garbage can icon next to a song represent in the latest (4.12.3) version of Limewire represent?

It means that file is junk.

You can add files as being junk by selecting that file & pressing the junk button. Here's some suggestions to add; 1. autogenerated spam results, & 2. autogenerated mp3 spam results & also 3. WARNING: Viruses on network you should be aware of! (click on link)

Viruses take much longer to filter out. The junk filter is a dynamic learning filter which needs to be trained. So it can sometimes take multiple junking before it takes the strongest effect. ;)