Taking a quick look at it, you should be able to do this multiple ways.
First Option:
Comment out or delete the line for "unoccupied.18" in on_five_year_pulse in 00_on_actions.txt
This should stop the ai from ever receiving the event at all.
Second Option:
Modify the beginning of event unoccupied.18 and change "ai = yes to "ai = no" in unoccupied_province_events.txt
This should stop the ai from being able to do the event even if other conditions to trigger are met.
Third Option:
Change "ai_chance = { factor = 33" to "ai_chance = { factor = 0" for the 'YES' option and "ai_chance = { factor = 66" to "ai_chance = { factor = 100" for the 'NO' option in unoccupied_province_events.txt
This should make it so the AI can only choose the 'NO' option no matter what.
Fourth Option:
Change the monthly_income and wealth to 9999 for the colonisation cost in unoccupied.18 in unoccupied_province_events.txt
This will simply make it so prohibitively expensive the ai will never have the money to colonise even when the event fires.
As always there's lots of ways to achieve the same result. I recommend going with the first option since it is certainly easiest.
Just put a # in front of line 1284 in A Game of Thrones\common\on_actions\00_on_actions.txt.
