Such adverts must result in interactions in the real world - either when delivering the service or buying the advert.
Why can't the law enforcement agencies treat the adverts as a honey pot? Surely their primary objective is to rescue the trafficked kids and arrest the pimps.
Blocking the adverts will just disperse it underground. The punters fuelling the trade will presumably still find their local source. Seems like a case of killing the messenger - so that the dirty linen stays out of sight and can be ignored.
"For Money Or Love" by Robin Lloyd did an investigation on the USA's newspaper and magazine adverts in 1975. It appears as if nothing has changed to fix the underlying social causes of poverty and abuse that drive people into the arms of the pimps.