Posts from How do I disable the starting bonus?
As an option, I can recommend setting it for the player in the editor, making it impossible to change, and then simply removing the bonus.
I'll take a look in the editor now, as I haven't been there in a while.
Added 3 minutes later
In the editor, in the Map Properties Tree:
>players>0..7
there will be
DefaultBonus
CanChangeBonus (set to false)