You cannot totally remove the threat of a determined terrorist on an aircraft. The very least you can do is minimise it. Better cockpit security is only part of the answer, as is better ground vetting of passengers. At least two sky marshalls, or in this country, gentlemen from Hereford, as has happened in the past (not always with the knowledge of the flight crew)is also effecive in reducing risk. There are effective ways to prevent passengers bringing metal knives aboard, but sadly in this day and age there are alternatives to metal blades if you have the connevtions and the money, which these terrorists obviously do and as any US policeman who has been to a Danny Innocento demonstration will attest, a knife is more dangerous than a gun in an enclosed space.