In src/map/guild.c
on line 389:
if( sd->status.guild_id )
{// already in a guild
clif_guild_created(sd,1);
return 0;
}
if( battle_config.guild_emperium_check && pc_search_inventory(sd,714) == -1 )
{// item required
714 is Emperium you can replace that so every time they try /guild it won't work because the item requied is different replace it with some not used.
then recompile.