Because at the end of the day it's still up to individuals to
whether to take fast food or not
So what if the ad has been banned, will it stop the people from
taking fast food? (Maybe, to a small extent only, since people will
not get reminded on the ads which made them patronise the fast food
joints. BUT, fastfood has sort of become part of our lifestyle,
since young? We dont need to get reminded to think of fast
food)
The opponent might debate: The ads will encourage people to have
fast food bla bla.. and now with those rap intro in fast food ads
like Macdonald that might attract teengers.... Think bout how to
counter this.... also the opponents might show a statistics of
obese children so, think bout it 
Just take a walk into any shoppin centres you can find at least
ONE fast food joints. So even with the ban of ADS, do you think it
can stop people from taking fast food??
Since you're debating on the against side (against banning of
fast food ads), i guess it will be easier. 
