I always change the value at the start of descr_strat (in Data\world\maps\campaign\imperial_campaign):
Code:
; webbird, map created by IceTorque Roma Surrectum Release 1 Build 5.0
campaign imperial_campaign
options bi
playable
romans_julii
romans_brutii
romans_scipii
egypt
seleucid
carthage
parthia
gauls
germans
greek_cities
macedon
pontus
armenia
dacia
scythia
spain
thrace
numidia
end
unlockable
end
nonplayable
slave
britons
end
start_date 473 summer
end_date 800 summer
gladiator_uprising_disabled
night_battles_enabled
brigand_spawn_value 999
pirate_spawn_value 90
Notice the brigand_spawn_value, this is what you want to change, and the pirate one for lowering the appearance of pirates.