Here's how to block all non-limewire cliensts (solely for educational purpose):
Download the source code
Open the file core/com/limegroup/gnutella/filters/RequeryFilter.java
and edit this fuction (paste the three lines marked with ">>"):

private boolean allow(QueryRequest q) {
//Kill automated requeries from LW 2.3 and earlier.
byte[] guid=q.getGUID();
>> //Filter ALL none-LimeWire queries
>> if (!GUID.isLimeGUID(guid))
>> return false;
if (GUID.isLimeGUID(guid)) {
if (GUID.isLimeRequeryGUID(guid, 0) //LW 2.2.0-2.2.3
|| GUID.isLimeRequeryGUID(guid, 1)) { //LW 2.2.4-2.3.x
return false;
}
}
return true;
}

Compile and voilà, you are filtering all queries from non-limewire hosts, while at the same time being able to download from them. - I'm not encouraging anybody to do so, however (although it saves a lot of your bandwidth).

Blocking freeloaders is a lot more complicated and almost impossible. LimeWire's freeloader-blocking does NOT work except for browsers.