I don't know if setplayerprop #n,nickname; works online (It doesn't work offline), but anyway he could just use this in a system NPC if it does work:
NPC Code:
if ((created||timeout)&&isweapon) {
if (strlen(#g)>0) setplayerprop #n,#e(0,strlen(#n)-strlen(#g),#n);
timeout=0.05;
}
Which would basically remove the guild tag from a player's name if he/she has one.