I think if you change the max age for adoption in descr_campaign_db to an age lower than the minimum age, the adoptions will be disabled.
Code:
<family_tree>
<max_age uint="150"/>
<max_age_for_marriage_for_male uint="80"/>
<max_age_for_marriage_for_female uint="50"/>
<max_age_before_death uint="165"/>
<max_age_of_child uint="10"/>
<old_age uint="120"/>
<age_of_manhood uint="16"/>
<daughters_age_of_consent uint="16"/>
<daughters_retirement_age uint="40"/>
<age_difference_min int="-10"/>
<age_difference_max int="30"/>
<parent_to_child_min_age_diff uint="12"/>
<min_adoption_age uint="16"/>
<max_adoption_age uint="45"/>
<max_age_for_conception uint="50"/>
<age_of_manhood_close uint="14"/>
<max_number_of_children uint="4"/>
</family_tree>