you asked if it is possible to block this using a firewall? sure, everything is possible any "desktop-firewall" will not be enough though, as these packets are send to all other bearshare clients you connect to (i suppose), thus blocking all of them would deny communication at all. but you should be able to do this with a firewall looking into _each_ package (i think this is called stateful inspection then). this would require:
a)
you to know the common structure of these packets to set up according rules, and
b)
processor time - each packet needs to be inspected.

so if you are willing and able to set up something like that ... well, go ahead