The act of slapping an unsuspecting individual as hard as possible across their forehead and shouting 'Spam'.
This can be countered by holding a guarding hand across the forehead, if suspicious of an approaching attack. This led to the development of the 'Neckback' which followed a similar path as the 'Spam' but involved slapping the back of the neck. This is also known as MAPS - spam backwards, you see.
When a double guard was developed, involving a person holding both their forehead and the back of their neck, a third route of attack, the 'Lipblap', was unveiled. This involved slapping down at a persons mouth (when they were talking for best effect) causing them to sound stupid and make a wet blubber noise. The beauty of this third attack meant that even when guarding, there was always one route of attack open - although the attacker now had to be quick to find the ungauarded area, and (especially with the 'Lipblap') the confusion tended to result in a simple face punch.

Also referred to as Fod. To be fodded, get a fodding, etc. Evolved into Fodder. As in: You dirty fodder.

'Spam' originally started as a description for those with an abnormally large forehead, leading to the insult 'spamhead', and the aforementioned game of slapping peoples foreheads while shouting 'SPAM!' If done correctly, a rewarding slap noise could be achieved.

Which reminds me of a biology field trip, during which Drew Gatherer became attracted to a girl with a very large 'spam'. Her forehead was so large we dubbed her 'Cranium'. Kudos to whoever thought that one up...

A hard quick slap to the forehead was known as a 'pig slap' at our school. Genevieve, while "sick" at home one day, looked out the window and saw her dog (a shepard) face to face with a very large pig. The pig had apparently escaped from a nearby farm. My sister Genevieve, terrified that the pig would hurt her german shepard (which, had I been the twelve year old girl faced with this decision, would have left the dog to fend for itslef) ran up to the 200 pound animal and slapped it on the forehead. I suppose this startled the animal enough that it took off for the woods, never to be found (although the farmer did come around asking questions, and told my parents that Genevieve that she was lucky to still be standing). I, of course, began slapping her on the forehead on a regular basis and yelling 'pig slap!' Being older than me and quite a bit taller, she would beat me whenever I tried this. Somehow, we developed a compromise where it became rule to first gently touch the person's forehead before delivering the smack, giving them time to jerk away. If they did not jerk away (usually out of inexperience, having never heard of a pig slap before), you had every right to smack them for being too stupid to move, and no retaliation on their part was allowed.

Indeed, a fine parctice honed at school and thence brought into the home for use on unsuspecting siblings (but not your parents).
For a more punishing regime however, positioning yourself behind, or adjacent to your chosen victim allowed you to sustain the spam act so that it became a weighty 'push' to the forehead. An unspoken aim was to remove the spamee from their seat in a bungled backflip motion (if, of course, they were sitting down)

We had a similar game to this called Oh! which consisted of smacking each other under the chin whilst proclaiming oh! (Sounds actually like a wolf falling down a cliff shaft; "aoowwwhh") the smackee then becomes the smacker, trying to spend the rest of the lesson attacking others, whom would be guarding their chins. god knows what we must have looked like, several of us running round with the tops of our hands under our chins, going oh to one another!!