I managed to eliminate all enemy units in Africa. In the manual it says that you have to win several battle to control a colony box. When moving the cursor over a conquered enemy box it says "control could mean a loss of 2 economic points to Germany" and "numbers of battle to win: 2. Battle won: 0"
So I am not sure if I actually control the box and deprive the enemy of Economic points since it says "could". There is just in Dar es salaam that I seem to actually have complet control after I had a message "Belgium got # NW points for the conquest of Da es salaam"