Putting posters on takeaway packaging will not stop someone from buying a knife.
Knives, amongst a lot of other things are age restricted, so if someone is too young to buy one they will simply get one by other means.
People who can read know perfectly well it is illegal to carry a knife unless for a specific purpose such as part of your job. This means you must be on your way to or from your job to justify carrying it. If you are a chef you can't carry a carving knife if you are going shopping in Waitrose or Sainsbury's.
I think the best deterrent for anyone caught carrying a knife without a valid explanation is to take them to the autopsy of a knife crime victim. If they faint then wait until they wake up before continuing. If they carry a knife after that then lock them up for a long time.
But to be honest I think the only way to break gang rule is harsh sentencing with harsh conditions inside prisons. That's how the broke the gang mentality at the end of the 19th/beginning of 20th century. Prison life was very harsh in those days and being a member of a gang such as the High Rip or Cornermen could get you 7 years penal servitude without actually being proved guilty of a crime. Just being a member was justification in those days.
Posters on boxes won't solve anything except put money in the banks/pockets of the bosses doing the printing.
