Well you will have to download a "JDK" (for free) from
http://java.sun.com and you will need "ant" from
http://jakarta.apache.org/.
Download the complete source code from
http://www.limewire.org (you'll have to use a cvs client, further instructions on the homepage).
In the file core/limegroup/gnutella/SpamFilter.java you have to edit line 39:
Replace
boolean filterAdult=settings.getFilterAdult();
with
boolean filterAdult=true;
Compile LimeWire and create the .jar files.