Whenever I've given blood there have been numerous points at which I would have been able to say that I shouldn't be giving blood - for example, if I said I was in the middle of dental work they would tell me to come back when it was finished, if I'd taken various forms of medication or had a vaccination in the last few days, etc.
The "travelling in Africa / Europe (and eating meat) for more than 6 months" thing would certainly work for US citizens; other excuses would include having had growth hormone treatment as a child, hepatitis and various other forms of infection, a recent blood transfusion - "Yes, I think Superman must have given me a blood transfusion when he was cloning me!" - or the sheer "fact" of being a clone (as Clark claimed post the Bonny & Clyde episode) and presumably having weird frog DNA and stem cells in his blood.
